Know the three steps to the Cisco troubleshooting model and the function that each step performs.
The three steps to the Cisco troubleshooting model are gather symptoms, isolate the
problem, correct the problem, and repeat if necessary. Once a problem is resolved, documentation
should be updated.
Know the troubleshooting methodologies and how to use them.
These troubleshooting
methodologies are bottom-up, top-down, and divide-and-conquer. In addition to understanding
them, know when it is most appropriate to use each method.
Be able to apply the Cisco troubleshooting methodology to example situations.
Know how
to apply each step of the troubleshooting model in real-life scenarios. You should be able to
determine what step in a troubleshooting scenario is next in the series, and understand how to
correlate a task with the correct step in the process.
IT Certification CCIE,CCNP,CCIP,CCNA,CCSP,Cisco Network Optimization and Security Tips
Troubleshooting Methodology Summary
With the complexity of today’s networks, it is important to adhere to a troubleshooting model
to aid in efficiently and effectively isolating and resolving network problems.
Various methods of problem isolation and the troubleshooting method itself help administrators
pinpoint problem areas and foresee future trouble. Troubleshooting skills are gained
through experience. It is unreasonable to expect that you can jump in on your first network failure
and be able to solve it quickly. Experience is the best teacher. Following a problem-solving
model helps you to reach a timely solution to network failures. It helps to know your network,
but the “shooting-from-the-hip” style of troubleshooting is nowhere near as effective as a
methodical and logical proces
Using the three steps of the Cisco troubleshooting model in order is a clear, calculated, and
logical way to make a network run more smoothly. The three methods of problem isolation
(bottom-up, top-down, and divide-and-conquer) are more subjective, and it is up to each individual
to use the appropriate method for the problem that they are facing. It is important to document
changes so you have a trail of what was done on the network. Finally, it’s important to
reverse any network alterations that did not correct the problem.
to aid in efficiently and effectively isolating and resolving network problems.
Various methods of problem isolation and the troubleshooting method itself help administrators
pinpoint problem areas and foresee future trouble. Troubleshooting skills are gained
through experience. It is unreasonable to expect that you can jump in on your first network failure
and be able to solve it quickly. Experience is the best teacher. Following a problem-solving
model helps you to reach a timely solution to network failures. It helps to know your network,
but the “shooting-from-the-hip” style of troubleshooting is nowhere near as effective as a
methodical and logical proces
Using the three steps of the Cisco troubleshooting model in order is a clear, calculated, and
logical way to make a network run more smoothly. The three methods of problem isolation
(bottom-up, top-down, and divide-and-conquer) are more subjective, and it is up to each individual
to use the appropriate method for the problem that they are facing. It is important to document
changes so you have a trail of what was done on the network. Finally, it’s important to
reverse any network alterations that did not correct the problem.
Divide-and-Conquer Troubleshooting Approach
The
divide-and-conquer troubleshooting
approach allows you to select the specific layer (Data
Link, Network, or Transport) of the OSI model in which to begin troubleshooting. You make
your selection based on experience with similar problems in the past, along with the specific
symptoms of the current trouble. After selecting the layer you wish to start with, the next task
is to determine the direction of the problem by determining whether the problem exists at,
above, or below this layer. Most commonly this is done by studying output from the IOS commands
on the router or through analysis of the output of network management tools. Once the
direction of the problem is determined, you continue troubleshooting through the OSI model in
that direction until you isolate the difficulty.
Often you can check the first four layers (Physical through Transport) by using
the
traceroute
command.
divide-and-conquer troubleshooting
approach allows you to select the specific layer (Data
Link, Network, or Transport) of the OSI model in which to begin troubleshooting. You make
your selection based on experience with similar problems in the past, along with the specific
symptoms of the current trouble. After selecting the layer you wish to start with, the next task
is to determine the direction of the problem by determining whether the problem exists at,
above, or below this layer. Most commonly this is done by studying output from the IOS commands
on the router or through analysis of the output of network management tools. Once the
direction of the problem is determined, you continue troubleshooting through the OSI model in
that direction until you isolate the difficulty.
Often you can check the first four layers (Physical through Transport) by using
the
traceroute
command.
Troubleshooting by Layer
The earlier sections of this chapter explained a general troubleshooting methodology. When going
through this methodology, it is often helpful to approach the problem in a logical manner that leverages
the OSI model. Therefore, Cisco has started backing a model of troubleshooting that does just
that. This model has three distinct approaches: bottom-up, top-down, and divide-and-conquer.
through this methodology, it is often helpful to approach the problem in a logical manner that leverages
the OSI model. Therefore, Cisco has started backing a model of troubleshooting that does just
that. This model has three distinct approaches: bottom-up, top-down, and divide-and-conquer.
Top-Down Troubleshooting Approach
If you suspect that the problem lies in a piece of software, then
top-down troubleshooting
should be used. You start by testing the application and work down the OSI layers to find the
source of the problem. The challenge to this type of troubleshooting is that you need to check
all the user’s network applications in order to find the one that is causing the errors. This is a
potentially time-consuming troubleshooting method if there are a large number of applications
that could be the source of the trouble.
top-down troubleshooting
should be used. You start by testing the application and work down the OSI layers to find the
source of the problem. The challenge to this type of troubleshooting is that you need to check
all the user’s network applications in order to find the one that is causing the errors. This is a
potentially time-consuming troubleshooting method if there are a large number of applications
that could be the source of the trouble.
Bottom-Up Troubleshooting Approach
As the name implies, when you use the
bottom-up troubleshooting
approach, you start with the
bottom—the Physical layer of the OSI model—and work your way up to the top—the Application
layer. This approach is used when you suspect the problem is at the Physical layer, or when you
are troubleshooting a complex network problem. In these situations, ensuring that the core components
required for networking are in place can go a long way toward isolating the problem.
The downside to bottom-up troubleshooting is that it can require the checking of each interface
along the path to see if errors are occurring there. Depending on the length of the path from
the end points of the problem, this process can be very time-consuming. In these cases, determining
the most likely culprit based on the symptoms of the trouble can save a lot of time.
bottom-up troubleshooting
approach, you start with the
bottom—the Physical layer of the OSI model—and work your way up to the top—the Application
layer. This approach is used when you suspect the problem is at the Physical layer, or when you
are troubleshooting a complex network problem. In these situations, ensuring that the core components
required for networking are in place can go a long way toward isolating the problem.
The downside to bottom-up troubleshooting is that it can require the checking of each interface
along the path to see if errors are occurring there. Depending on the length of the path from
the end points of the problem, this process can be very time-consuming. In these cases, determining
the most likely culprit based on the symptoms of the trouble can save a lot of time.
Document the Changes
The network problem has been officially resolved after you’ve implemented a change, observed
that the symptoms have disappeared, and can successfully execute the tests that were used to aid
in gathering information about the problem. In this example, the way to verify that the problem
is solved is for Host A to try to ftp to Host Z. If this test is successful, then the problem is resolved.
In the previous sections, we have emphasized that documentation is an integral part of troubleshooting.
When you keep track of the alterations that were made, the routers, switches, or hosts that
were changed, and when the changes occurred, you have valuable information for future reference.
There is always the possibility that something you changed might have affected something else and
you didn’t notice it. If this happens, you will have documentation to refer to, so you can undo the
changes. Or if a similar problem occurs in the future, you can refer to these documents to resolve the
new problem, based on what was done the last time. Later chapters in this book will give you more
information about documentation and establishing baseline information.
that the symptoms have disappeared, and can successfully execute the tests that were used to aid
in gathering information about the problem. In this example, the way to verify that the problem
is solved is for Host A to try to ftp to Host Z. If this test is successful, then the problem is resolved.
In the previous sections, we have emphasized that documentation is an integral part of troubleshooting.
When you keep track of the alterations that were made, the routers, switches, or hosts that
were changed, and when the changes occurred, you have valuable information for future reference.
There is always the possibility that something you changed might have affected something else and
you didn’t notice it. If this happens, you will have documentation to refer to, so you can undo the
changes. Or if a similar problem occurs in the future, you can refer to these documents to resolve the
new problem, based on what was done the last time. Later chapters in this book will give you more
information about documentation and establishing baseline information.
Looks Can Be Deceiving
One common mistake when observing the results of a change is seeing symptoms go away
and assuming that the problem has been solved. For example, assume that users are complaining
about slow response time while accessing the Internet. In the course of troubleshooting,
you find and correct some non-optimally-configured interface settings on the
router on the users’ segment. You then go back to the user who originally reported the problem.
She reports that everything is running fine now. However, she neglects to mention the
fact that there was a shift change, and now only two people are connecting to the Internet
where there used to be 50. The next day, when all of the users are back online, the problem
repeats itself. If an analysis of the observations had been done, it would have demonstrated
that the traffic flow to the Internet had dropped off and that this could be a contributing factor
to the improvement in response times.
As is demonstrated in this example, failure to analyze your observations creates the risk that
important information can be overlooked and the problem will recur. To avoid this possibility,
make sure to look at the entire scope of the problem. Use your network management tools to
help you determine whether the problem is really resolved. You can also look at your network
baseline information to find out what the “normal” traffic pattern looks like. In this example, it
should show a sharp drop-off in utilization when the shift changes. This would tell you that the
improvement in connection speed may not be due to the interface changes you’ve made, but
rather due to a lower volume of traffic. More verification may be needed.
and assuming that the problem has been solved. For example, assume that users are complaining
about slow response time while accessing the Internet. In the course of troubleshooting,
you find and correct some non-optimally-configured interface settings on the
router on the users’ segment. You then go back to the user who originally reported the problem.
She reports that everything is running fine now. However, she neglects to mention the
fact that there was a shift change, and now only two people are connecting to the Internet
where there used to be 50. The next day, when all of the users are back online, the problem
repeats itself. If an analysis of the observations had been done, it would have demonstrated
that the traffic flow to the Internet had dropped off and that this could be a contributing factor
to the improvement in response times.
As is demonstrated in this example, failure to analyze your observations creates the risk that
important information can be overlooked and the problem will recur. To avoid this possibility,
make sure to look at the entire scope of the problem. Use your network management tools to
help you determine whether the problem is really resolved. You can also look at your network
baseline information to find out what the “normal” traffic pattern looks like. In this example, it
should show a sharp drop-off in utilization when the shift changes. This would tell you that the
improvement in connection speed may not be due to the interface changes you’ve made, but
rather due to a lower volume of traffic. More verification may be needed.
Step 1: Gather Symptoms
As you can see, the user’s problem is vague; you need more information if you are to solve the
problem any time soon. This is where the first step comes in. Gathering symptoms is the step in
the troubleshooting model when details about the problem are gathered from as many sources
as is practical. These symptoms can come from a number of sources, including but not limited
to the network devices, users, monitoring tools, and console messages.
Now, while you still have the user on the line, the first step is to ask him what he means when
he says he can’t “get to” Host Z. The user then defines the situation by telling you that he can’t
ftp to Host Z. Ask the user if he experiences any other difficulties or if this is the only one. Verify
where the user is currently located. After these preliminary questions, you’ll have a basic idea of
what is and isn’t working. Unfortunately, you can’t simply assume that FTP is broken, because
there are many other pieces of the network that can contribute to this problem.
At this point, the problem is still pretty vague and needs more definition. Additional information
should include data that excludes other possibilities and helps pinpoint the actual problem.
An example in the case we’re discussing is to verify whether you can ping, traceroute, or
telnet to Host Z, thus reducing the number of possible causes.
Depending on the user and situation, you may or may not be able to get more detailed information.
It is up to you as a network engineer or administrator to solve the problem, which
means that you may have to get the information yourself.
It is important that you gain as much information as possible to actually define the problem
correctly. Without a proper and specific definition of the problem, it will be much harder to isolate
and resolve. Information that is useful for gathering symptoms is listed in Table 33.1.
TABLE 3 3 . 1
Useful Information for Gathering Symptoms
Information Example
Symptoms Can’t telnet, ftp, or get to the WWW.
Reproducibility Is this a one-time occurrence, or does it always happen?
Timeline When did it start? How long did it last? How often does it occur? Has the
current configuration ever worked properly?
Scope What are you able to access successfully via Telnet or FTP? Which WWW
sites can you reach, if any? Who else does this affect?
Baseline Info Were any recent changes made to the network configurations?
All of this information can be used to guide you to the actual problem and to create the problem
statement. Use your network topology diagram and check each item in Table 33.1. Once
you are done talking to the user, you need to define what is working and what isn’t.
Figure 33.4 is a picture of your network. Although the large X on the Frame Relay cloud represents
that there is an FTP connectivity issue, it does not indicate the location of the failure.
Right now, all you know is that a single user cannot ftp to Host Z.
Reproduce the Problem
Before spending time and effort trying to solve this problem, verify that it is still a problem.
Troubleshooting is a waste of time and resources if the problem can’t be reproduced. It’s just
like a dog chasing its tail. If the issue is intermittent, further steps should be taken to capture as
much information as possible about the event the next time it does occur. This will help narrow
down the scope of items you will look at.
Understand the Timeline
In addition to verifying whether the problem is reproducible, it is important to investigate the
frequency of the problem. For instance, maybe it happens only once or twice a day. By establishing
a timeframe you can more readily identify any possible causes. In addition, you need to
know whether this is the first time the user has attempted this function. There is a different set
of variables involved with an item that worked yesterday but not today than there is with something
that fails during first-time use. Obviously, if it worked yesterday, you can look at what
changed overnight and look for something that is broken. If the user has never used this feature
before, there may be an existing access list or other security device that has only now been activated
by the user’s initial use of this application.
Determine the Scope of a Problem
Next, you need to find out whether anyone else is unable to ftp to Host Z. If others can ftp to Host Z
(for the sake of this example, assume that they can), you can be pretty sure that the problem is
specific to the user, either on their station or on the destination host. This step determines the scope
of the problem and helps to differentiate between a user-specific problem and a more widely spread
problem. Figure 33.5 shows that other hosts can ftp to Host Z without any problems.
problem any time soon. This is where the first step comes in. Gathering symptoms is the step in
the troubleshooting model when details about the problem are gathered from as many sources
as is practical. These symptoms can come from a number of sources, including but not limited
to the network devices, users, monitoring tools, and console messages.
Now, while you still have the user on the line, the first step is to ask him what he means when
he says he can’t “get to” Host Z. The user then defines the situation by telling you that he can’t
ftp to Host Z. Ask the user if he experiences any other difficulties or if this is the only one. Verify
where the user is currently located. After these preliminary questions, you’ll have a basic idea of
what is and isn’t working. Unfortunately, you can’t simply assume that FTP is broken, because
there are many other pieces of the network that can contribute to this problem.
At this point, the problem is still pretty vague and needs more definition. Additional information
should include data that excludes other possibilities and helps pinpoint the actual problem.
An example in the case we’re discussing is to verify whether you can ping, traceroute, or
telnet to Host Z, thus reducing the number of possible causes.
Depending on the user and situation, you may or may not be able to get more detailed information.
It is up to you as a network engineer or administrator to solve the problem, which
means that you may have to get the information yourself.
It is important that you gain as much information as possible to actually define the problem
correctly. Without a proper and specific definition of the problem, it will be much harder to isolate
and resolve. Information that is useful for gathering symptoms is listed in Table 33.1.
TABLE 3 3 . 1
Useful Information for Gathering Symptoms
Information Example
Symptoms Can’t telnet, ftp, or get to the WWW.
Reproducibility Is this a one-time occurrence, or does it always happen?
Timeline When did it start? How long did it last? How often does it occur? Has the
current configuration ever worked properly?
Scope What are you able to access successfully via Telnet or FTP? Which WWW
sites can you reach, if any? Who else does this affect?
Baseline Info Were any recent changes made to the network configurations?
All of this information can be used to guide you to the actual problem and to create the problem
statement. Use your network topology diagram and check each item in Table 33.1. Once
you are done talking to the user, you need to define what is working and what isn’t.
Figure 33.4 is a picture of your network. Although the large X on the Frame Relay cloud represents
that there is an FTP connectivity issue, it does not indicate the location of the failure.
Right now, all you know is that a single user cannot ftp to Host Z.
Reproduce the Problem
Before spending time and effort trying to solve this problem, verify that it is still a problem.
Troubleshooting is a waste of time and resources if the problem can’t be reproduced. It’s just
like a dog chasing its tail. If the issue is intermittent, further steps should be taken to capture as
much information as possible about the event the next time it does occur. This will help narrow
down the scope of items you will look at.
Understand the Timeline
In addition to verifying whether the problem is reproducible, it is important to investigate the
frequency of the problem. For instance, maybe it happens only once or twice a day. By establishing
a timeframe you can more readily identify any possible causes. In addition, you need to
know whether this is the first time the user has attempted this function. There is a different set
of variables involved with an item that worked yesterday but not today than there is with something
that fails during first-time use. Obviously, if it worked yesterday, you can look at what
changed overnight and look for something that is broken. If the user has never used this feature
before, there may be an existing access list or other security device that has only now been activated
by the user’s initial use of this application.
Determine the Scope of a Problem
Next, you need to find out whether anyone else is unable to ftp to Host Z. If others can ftp to Host Z
(for the sake of this example, assume that they can), you can be pretty sure that the problem is
specific to the user, either on their station or on the destination host. This step determines the scope
of the problem and helps to differentiate between a user-specific problem and a more widely spread
problem. Figure 33.5 shows that other hosts can ftp to Host Z without any problems.
Cisco Troubleshooting Model
Imagine trying to solve a network failure by using a different approach every time. With today’s
complex networks, the possible scenarios would be innumerable. Because so many different
things can go wrong within a network, it’s possible to start from many different points. Not
only is this an ineffective method of troubleshooting, but it’s also time-consuming, and time is
very valuable in a “network down” situation.
Cisco has designed an effective
troubleshooting model
that contains three steps. A troubleshooting
model is a list of troubleshooting steps or processes that can be followed to provide an
efficient manner of resolving network problems. The headings in this section contain information
specific to each step of the troubleshooting model. After the three steps are completed and the
problem is resolved, a few more actions follow, such as documenting the problem-solving events.
To be effective when troubleshooting and to achieve faster resolution times, follow the model
outlined in Figure 33.2. This flow chart shows the three steps.
FIGURE 3 3 . 2
Cisco’s troubleshooting model
Gather Symptoms Isolate Problem Correct Problem
Update
Documentation
Troubleshooting
Complete
Corrected
Yes
The troubleshooting process begins when a network failure is reported to you. The following
are brief descriptions of the steps to take:
1.
Gather symptoms.
At this point in the process, it is important gather and document the
symptoms of the problem that is being experienced.
2.
Isolate the problem.
After identifying the symptoms, the administrator looks for commonalities
in the symptoms and tries to determine at what layer of the OSI model the problem
is occurring. During this phase, it may be necessary to go back and gather more symptoms.
3.
Correct the problem.
Based on the information that was gathered and the determinations that
were made in the previous two steps, the network administrator now makes the changes necessary
to correct the problem. Once the corrective steps have been taken, the administrator
observes the results of the changes to ensure that the problem was corrected. If the problem
was not corrected, then the changes made should be backed out and the administrator should
start the troubleshooting process over with the gather symptoms stage. If the changes do correct
the problem, then the administrator should update the necessary documentation. The
final item of importance when correcting the problem is to make sure that you make only one
change at a time. This will ensure that you do not make unnecessary changes, which could
introduce new problems.
The best way to understand how Cisco’s model works and how you should use it is by looking
at an example. For this example, assume you are in charge of operational support of the network
pictured in Figure 33.3. There are two campus networks, connected via a Frame Relay
cloud. Within each network, VLANs are connected to a Catalyst 6500 switch and then to a core
router that has a connection to the Frame Relay cloud in one way or another.
complex networks, the possible scenarios would be innumerable. Because so many different
things can go wrong within a network, it’s possible to start from many different points. Not
only is this an ineffective method of troubleshooting, but it’s also time-consuming, and time is
very valuable in a “network down” situation.
Cisco has designed an effective
troubleshooting model
that contains three steps. A troubleshooting
model is a list of troubleshooting steps or processes that can be followed to provide an
efficient manner of resolving network problems. The headings in this section contain information
specific to each step of the troubleshooting model. After the three steps are completed and the
problem is resolved, a few more actions follow, such as documenting the problem-solving events.
To be effective when troubleshooting and to achieve faster resolution times, follow the model
outlined in Figure 33.2. This flow chart shows the three steps.
FIGURE 3 3 . 2
Cisco’s troubleshooting model
Gather Symptoms Isolate Problem Correct Problem
Update
Documentation
Troubleshooting
Complete
Corrected
Yes
The troubleshooting process begins when a network failure is reported to you. The following
are brief descriptions of the steps to take:
1.
Gather symptoms.
At this point in the process, it is important gather and document the
symptoms of the problem that is being experienced.
2.
Isolate the problem.
After identifying the symptoms, the administrator looks for commonalities
in the symptoms and tries to determine at what layer of the OSI model the problem
is occurring. During this phase, it may be necessary to go back and gather more symptoms.
3.
Correct the problem.
Based on the information that was gathered and the determinations that
were made in the previous two steps, the network administrator now makes the changes necessary
to correct the problem. Once the corrective steps have been taken, the administrator
observes the results of the changes to ensure that the problem was corrected. If the problem
was not corrected, then the changes made should be backed out and the administrator should
start the troubleshooting process over with the gather symptoms stage. If the changes do correct
the problem, then the administrator should update the necessary documentation. The
final item of importance when correcting the problem is to make sure that you make only one
change at a time. This will ensure that you do not make unnecessary changes, which could
introduce new problems.
The best way to understand how Cisco’s model works and how you should use it is by looking
at an example. For this example, assume you are in charge of operational support of the network
pictured in Figure 33.3. There are two campus networks, connected via a Frame Relay
cloud. Within each network, VLANs are connected to a Catalyst 6500 switch and then to a core
router that has a connection to the Frame Relay cloud in one way or another.
The Complexity of Internetworks
When a network failure occurs, time is of the essence. When a production network goes down,
several things are affected. The most important of these is the bottom line—network failures
cost money.
A good example is a call-center network. The company relies on the network to be available
for its employees so that they can take phone orders, answer inquiries, or perform other
business transactions that generate income. A failure in this environment needs to be diagnosed
and repaired in a timely manner. The longer the network is down, the more money the
company loses.
To minimize monetary and productivity losses, network failures must be resolved quickly.
Troubleshooting is an integral part of getting this done. Intimate knowledge of a network also
facilitates rapid resolution. Armed with a few troubleshooting skills and intimate knowledge of
your network, you can solve most problems rather quickly, thus saving money.
Hold on a minute. What if you’re new on the job and you don’t yet have an intimate
knowledge of the network? You can probably get up to speed quickly enough, right?
Although that may have been the case in the past, getting up to speed becomes an overwhelming
challenge in today’s complex networks. These networks consist of many facets of
routing, dial-up, switching, video, WAN (ISDN, Frame Relay, ATM, and others), LAN, and
VLAN technologies.
Figure 33.1 gives you an idea of how these technologies intertwine. Notice that ATM, Frame
Relay, Token Ring, Ethernet, and FDDI all are present. Each technology has its own properties
and commands to allow for troubleshooting. Various protocols are used for each of these technologies.
In addition, different applications require specific network resources. (At least the
seven-layer OSI model, which you will review in Chapter 36, “Protocol Attributes,” is used to
maintain a common template when designing new technologies and protocols.) It would take
you a long time to master all of the technologies implemented in the network and to be able to
solve network problems based on your knowledge of the network alone. All of these factors
contribute to today’s complex network environments.
There must be an easier, more logical way to efficiently and successfully troubleshoot without
having to become intimately familiar with every network environment. Well, you’ll be
happy to know that there is an easier option—following a troubleshooting model, which is discussed
in detail in this chapter. By following a troubleshooting model, the need for intimate
knowledge of the network is reduced. A troubleshooting model should be adopted to help
resolve network malfunctions and reduce downtime.
Let’s move on to discuss Cisco’s model in detail.
several things are affected. The most important of these is the bottom line—network failures
cost money.
A good example is a call-center network. The company relies on the network to be available
for its employees so that they can take phone orders, answer inquiries, or perform other
business transactions that generate income. A failure in this environment needs to be diagnosed
and repaired in a timely manner. The longer the network is down, the more money the
company loses.
To minimize monetary and productivity losses, network failures must be resolved quickly.
Troubleshooting is an integral part of getting this done. Intimate knowledge of a network also
facilitates rapid resolution. Armed with a few troubleshooting skills and intimate knowledge of
your network, you can solve most problems rather quickly, thus saving money.
Hold on a minute. What if you’re new on the job and you don’t yet have an intimate
knowledge of the network? You can probably get up to speed quickly enough, right?
Although that may have been the case in the past, getting up to speed becomes an overwhelming
challenge in today’s complex networks. These networks consist of many facets of
routing, dial-up, switching, video, WAN (ISDN, Frame Relay, ATM, and others), LAN, and
VLAN technologies.
Figure 33.1 gives you an idea of how these technologies intertwine. Notice that ATM, Frame
Relay, Token Ring, Ethernet, and FDDI all are present. Each technology has its own properties
and commands to allow for troubleshooting. Various protocols are used for each of these technologies.
In addition, different applications require specific network resources. (At least the
seven-layer OSI model, which you will review in Chapter 36, “Protocol Attributes,” is used to
maintain a common template when designing new technologies and protocols.) It would take
you a long time to master all of the technologies implemented in the network and to be able to
solve network problems based on your knowledge of the network alone. All of these factors
contribute to today’s complex network environments.
There must be an easier, more logical way to efficiently and successfully troubleshoot without
having to become intimately familiar with every network environment. Well, you’ll be
happy to know that there is an easier option—following a troubleshooting model, which is discussed
in detail in this chapter. By following a troubleshooting model, the need for intimate
knowledge of the network is reduced. A troubleshooting model should be adopted to help
resolve network malfunctions and reduce downtime.
Let’s move on to discuss Cisco’s model in detail.
Step 3: Correct the Problem
The investigation gave you three leads about the source of the problem. Now it’s a matter of
checking out each possibility and determining which one is most likely the source of the issue.
The majority of the possibilities point directly at the host machine, so start there. The first
two causes are host configuration issues. Now, assume that you’ve checked the TCP/IP configuration
on the host and everything is configured properly. You can eliminate the host machine
as the culprit.
You then move on to the remaining possible cause, which is an access list on the router. While
looking at the configuration on the router, you see that an access list is applied to the Ethernet
interface directly connected to the host segment. After reviewing the syntax of the access list, you
determine that it is the cause of the failure.
Great—you’ve found the problem. Now what? Once you find the problem, you must decide
what is needed to fix it. In this case, it is an access-list problem, so there are some special considerations
about how to restore functionality. You must be careful in your actions here, because that
access list may contain other entries that provide security or other network administrative functionality.
You can’t just remove the list—you could cause new problems as you fix the original one.
The best thing to do in this situation is to make a copy of the access list in a text editor,
and then make changes that are specific to your problem. When editing the access list, change
its number. After all of the changes are made in your text editor, ensure that you have a current
backup of the configuration on the router in case you need to restore the original configuration.
Then paste the modified access list back into the router. Finally, go to the interface
and apply the new access list. By following this procedure, the access list is never removed
from the interface.
Obviously, you have now changed the access-list number that is applied to the interface, so
any documentation that refers to the original number will need to be updated. If the access list
that was causing the problem was applied only to Ethernet 0, you can now safely remove
the old list, update this list with the corrections to address your problem, and put it back on the
router. Then reapply this list to Ethernet 0. As was the case before, the access list is never
removed from the interface.
When you are going through the troubleshooting methodology, it is important that you
don’t fix one problem and cause another. Before implementing any changes, think it through or
discuss it with coworkers to pick it apart, and make sure that your solution will fix the problem
without doing anything to create adverse side effects.
Another good practice when implementing changes is to change only one thing at a time, if
possible. If multiple changes must be made, it is best to make the changes in small sets. This way
it is easier to keep track of what was done, what worked, and what didn’t. Observing the effects
of a change becomes much more effective if only a single change is made at a time. There is nothing
worse than troubleshooting your self-induced errors in addition to the original difficulties!
To summarize, follow these practices and guidelines to making changes:
Make one change or a set of related changes at a time, and then observe the results.
Make non-impacting changes—this means trying not to cause other problems while implementing
the changes. The more transparent the change, the better.
Do not create security holes when changing access lists, TACACS+, RADIUS, or other
security-oriented configurations.
Most importantly, make sure you can revert to the original configuration if unforeseen problems
occur as a result of the change. Always have a backup or copy of the configuration.
In the preceding paragraphs, there were references to observing the results of the changes.
Observing results consists of using the exact same methods and commands that were used to
obtain information to gather symptoms—to see whether the changes you implemented had the
results you want. By making a change and then testing its effectiveness, you move toward the
correct solution.
It may take one or more changes to fix the problem, but you should observe each change separately
to monitor progress and to make sure that the alteration doesn’t create any adverse
effects. After the first change is made, you should be able to gather enough information to learn
whether or not the modification was effective, even if it doesn’t entirely solve the problem.
If the changes made have corrected the problem, move on and document the modifications
that were made to the network. If the changes did not work, you need to go back and either
gather more information or try another one of the potential issues that you identified while
isolating the problem.
Iterations—repetitions of certain steps within the troubleshooting model—are simply ways
of whittling away at a larger problem. By implementing changes and monitoring the results, you
can move toward solving the overall problem.
Iterations of the troubleshooting process allow you to focus with more and more detail on
the possible causes of the failure. The result of focusing on the problem is your ability to identify
more specific possibilities for the failure.
The iteration process has its own set of steps: While working through the process, you might
get more ideas of possible sources of the trouble. Write them down; if the current changes do
not work, you have notes about some other options. If you feel that you have exhausted all of
the possible causes, you should probably go back and gather more information. You will probably
find additional clues.
This is also the time to undo any changes that had adverse effects or that did not fix the problem.
Make sure to document what was done, so it will be easier to undo the any configuration
modifications.
checking out each possibility and determining which one is most likely the source of the issue.
The majority of the possibilities point directly at the host machine, so start there. The first
two causes are host configuration issues. Now, assume that you’ve checked the TCP/IP configuration
on the host and everything is configured properly. You can eliminate the host machine
as the culprit.
You then move on to the remaining possible cause, which is an access list on the router. While
looking at the configuration on the router, you see that an access list is applied to the Ethernet
interface directly connected to the host segment. After reviewing the syntax of the access list, you
determine that it is the cause of the failure.
Great—you’ve found the problem. Now what? Once you find the problem, you must decide
what is needed to fix it. In this case, it is an access-list problem, so there are some special considerations
about how to restore functionality. You must be careful in your actions here, because that
access list may contain other entries that provide security or other network administrative functionality.
You can’t just remove the list—you could cause new problems as you fix the original one.
The best thing to do in this situation is to make a copy of the access list in a text editor,
and then make changes that are specific to your problem. When editing the access list, change
its number. After all of the changes are made in your text editor, ensure that you have a current
backup of the configuration on the router in case you need to restore the original configuration.
Then paste the modified access list back into the router. Finally, go to the interface
and apply the new access list. By following this procedure, the access list is never removed
from the interface.
Obviously, you have now changed the access-list number that is applied to the interface, so
any documentation that refers to the original number will need to be updated. If the access list
that was causing the problem was applied only to Ethernet 0, you can now safely remove
the old list, update this list with the corrections to address your problem, and put it back on the
router. Then reapply this list to Ethernet 0. As was the case before, the access list is never
removed from the interface.
When you are going through the troubleshooting methodology, it is important that you
don’t fix one problem and cause another. Before implementing any changes, think it through or
discuss it with coworkers to pick it apart, and make sure that your solution will fix the problem
without doing anything to create adverse side effects.
Another good practice when implementing changes is to change only one thing at a time, if
possible. If multiple changes must be made, it is best to make the changes in small sets. This way
it is easier to keep track of what was done, what worked, and what didn’t. Observing the effects
of a change becomes much more effective if only a single change is made at a time. There is nothing
worse than troubleshooting your self-induced errors in addition to the original difficulties!
To summarize, follow these practices and guidelines to making changes:
Make one change or a set of related changes at a time, and then observe the results.
Make non-impacting changes—this means trying not to cause other problems while implementing
the changes. The more transparent the change, the better.
Do not create security holes when changing access lists, TACACS+, RADIUS, or other
security-oriented configurations.
Most importantly, make sure you can revert to the original configuration if unforeseen problems
occur as a result of the change. Always have a backup or copy of the configuration.
In the preceding paragraphs, there were references to observing the results of the changes.
Observing results consists of using the exact same methods and commands that were used to
obtain information to gather symptoms—to see whether the changes you implemented had the
results you want. By making a change and then testing its effectiveness, you move toward the
correct solution.
It may take one or more changes to fix the problem, but you should observe each change separately
to monitor progress and to make sure that the alteration doesn’t create any adverse
effects. After the first change is made, you should be able to gather enough information to learn
whether or not the modification was effective, even if it doesn’t entirely solve the problem.
If the changes made have corrected the problem, move on and document the modifications
that were made to the network. If the changes did not work, you need to go back and either
gather more information or try another one of the potential issues that you identified while
isolating the problem.
Iterations—repetitions of certain steps within the troubleshooting model—are simply ways
of whittling away at a larger problem. By implementing changes and monitoring the results, you
can move toward solving the overall problem.
Iterations of the troubleshooting process allow you to focus with more and more detail on
the possible causes of the failure. The result of focusing on the problem is your ability to identify
more specific possibilities for the failure.
The iteration process has its own set of steps: While working through the process, you might
get more ideas of possible sources of the trouble. Write them down; if the current changes do
not work, you have notes about some other options. If you feel that you have exhausted all of
the possible causes, you should probably go back and gather more information. You will probably
find additional clues.
This is also the time to undo any changes that had adverse effects or that did not fix the problem.
Make sure to document what was done, so it will be easier to undo the any configuration
modifications.
Centralized Security in Remote Access Networks Exam Essentials
Understand the components of AAA. You should know that AAA is the acronym for
authentication, authorization, and accounting. Authentication is used to verify a user’s
authenticity, usually with a username and password. Authorization is used to determine
which services are available to a verified user. Accounting is used to audit the user’s activity
on the system to provide tracking.
Know the services provided by CiscoSecure. The CiscoSecure software runs on Windows NT and
Unix and provides a Java-based web client for configuration. The software provides RADIUS
and TACACS+ services for authentication, authorization, and accounting. The software can
store and retrieve user information with outside databases, including Oracle and Sybase.
Understand the functions provided by each AAA component, including the six accounting types.
In addition to the AAA functions of authorizing and authenticating a user for access to various functions
in the router, the accounting function can audit commands, connections, EXEC, network, system,
and resources.
Know how to configure AAA services for Cisco IOS. AAA has been updated since its initial
inception; the command aaa new-model is used so the user can utilize the new AAA commands.
There are many AAA commands used to configure authentication, authorization, and accounting
on a Cisco device. Each service command begins with the aaa prefix. You don’t need to
know the AAA commands for Cisco Catalyst series switches, but they are included in this chapter
for completeness.
Understand the differences between packet-mode and character-mode services. Packet-mode
services are typically dial-up connections, including asynchronous and ISDN access. Characterbased
services are connections such as login, exec, NASI, and commands. Most of these services
terminate at the access device, which is typical of character-mode services.
Know that aaa new-model requires additional commands to configure correctly. Invoking the
aaa new-model command with no other parameters will lock the administrator out of the router.
authentication, authorization, and accounting. Authentication is used to verify a user’s
authenticity, usually with a username and password. Authorization is used to determine
which services are available to a verified user. Accounting is used to audit the user’s activity
on the system to provide tracking.
Know the services provided by CiscoSecure. The CiscoSecure software runs on Windows NT and
Unix and provides a Java-based web client for configuration. The software provides RADIUS
and TACACS+ services for authentication, authorization, and accounting. The software can
store and retrieve user information with outside databases, including Oracle and Sybase.
Understand the functions provided by each AAA component, including the six accounting types.
In addition to the AAA functions of authorizing and authenticating a user for access to various functions
in the router, the accounting function can audit commands, connections, EXEC, network, system,
and resources.
Know how to configure AAA services for Cisco IOS. AAA has been updated since its initial
inception; the command aaa new-model is used so the user can utilize the new AAA commands.
There are many AAA commands used to configure authentication, authorization, and accounting
on a Cisco device. Each service command begins with the aaa prefix. You don’t need to
know the AAA commands for Cisco Catalyst series switches, but they are included in this chapter
for completeness.
Understand the differences between packet-mode and character-mode services. Packet-mode
services are typically dial-up connections, including asynchronous and ISDN access. Characterbased
services are connections such as login, exec, NASI, and commands. Most of these services
terminate at the access device, which is typical of character-mode services.
Know that aaa new-model requires additional commands to configure correctly. Invoking the
aaa new-model command with no other parameters will lock the administrator out of the router.
Virtual ProfilesVirtual Profiles
Virtual profiles and virtual templates provide ways to apply centralized, user-specific
parameters to multiple access servers and their physical interfaces. This can greatly reduce
the impact of changes to widely distributed access points.
As suggested by the name, there is a difference between a virtual profile and the element it
replaces—the dialer profile. Dialer profiles maintain information on a single access server for
specific users. The virtual profile adds the following:
User-specific configurations served from the AAA server
An open methodology for defining both standards-based and vendor-specific parameters
After the user authenticates the system, a virtual template is applied to the virtual access
interface. User parameters are then obtained from the AAA server (security server) and applied
to the virtual access interface. This solution allows for better scalability and easier administration
than would be allowed with standard dialer profiles. As a result, the virtual profile is actually
a combination of the physical interface, generic information stored in a virtual template on
the access server, and user-specific parameters stored on the security server.
parameters to multiple access servers and their physical interfaces. This can greatly reduce
the impact of changes to widely distributed access points.
As suggested by the name, there is a difference between a virtual profile and the element it
replaces—the dialer profile. Dialer profiles maintain information on a single access server for
specific users. The virtual profile adds the following:
User-specific configurations served from the AAA server
An open methodology for defining both standards-based and vendor-specific parameters
After the user authenticates the system, a virtual template is applied to the virtual access
interface. User parameters are then obtained from the AAA server (security server) and applied
to the virtual access interface. This solution allows for better scalability and easier administration
than would be allowed with standard dialer profiles. As a result, the virtual profile is actually
a combination of the physical interface, generic information stored in a virtual template on
the access server, and user-specific parameters stored on the security server.
Step 2: Isolate the Problem
This step within the troubleshooting model is used to contemplate the possible causes of the failure.
Obviously, it is quite easy to create a very long list of possible causes. That’s why it’s so
important to gather as much relevant information as you can in the gathering symptoms phase.
By defining the problem and assigning the corresponding boundaries, the resulting list of possible
causes diminishes because the entries in the list will be focused on the actual problem and
not on “possible” problems.
First, review what you know about your sample problem:
Host A can’t ftp to Host Z.
Host A can’t ftp to any host on Campus B.
Host A can’t ping to anywhere outside its own network.
Host A can ftp to any host on its own network.
All other hosts on Host A’s network can ftp to Host Z, as well as to other hosts.
Based on what you know, you now need to list possible causes. These possible causes are
as follows:
No default gateway is configured on Host A.
The wrong subnet mask is configured.
There is a misconfigured access list on the router connected to the switch on Campus A.
If you had not gathered such specific information in step 1, this list could have included all
possible problems with any piece of equipment between Host A and Host Z. That would have
been a long list, and it would take a lot of time to eliminate all of the possible causes.
Remember that because these are only
possible
causes, you still have to choose the most
likely option, implement it, and observe to see whether the changes made were effective. When
the list of possible problems is long, it may require more iterations of the problem-solving steps
to actually solve the problem. In this example, you have only three possible causes, so this is a
much more manageable list. Although there may be other possible causes that you can think of
(and it’s great that you can do that), for this example and in the interest of simplicity, only these
three are listed.
Here’s where it gets interesting. You now have to check each of these possibilities and fix
them if they are the cause of the problem.
Obviously, it is quite easy to create a very long list of possible causes. That’s why it’s so
important to gather as much relevant information as you can in the gathering symptoms phase.
By defining the problem and assigning the corresponding boundaries, the resulting list of possible
causes diminishes because the entries in the list will be focused on the actual problem and
not on “possible” problems.
First, review what you know about your sample problem:
Host A can’t ftp to Host Z.
Host A can’t ftp to any host on Campus B.
Host A can’t ping to anywhere outside its own network.
Host A can ftp to any host on its own network.
All other hosts on Host A’s network can ftp to Host Z, as well as to other hosts.
Based on what you know, you now need to list possible causes. These possible causes are
as follows:
No default gateway is configured on Host A.
The wrong subnet mask is configured.
There is a misconfigured access list on the router connected to the switch on Campus A.
If you had not gathered such specific information in step 1, this list could have included all
possible problems with any piece of equipment between Host A and Host Z. That would have
been a long list, and it would take a lot of time to eliminate all of the possible causes.
Remember that because these are only
possible
causes, you still have to choose the most
likely option, implement it, and observe to see whether the changes made were effective. When
the list of possible problems is long, it may require more iterations of the problem-solving steps
to actually solve the problem. In this example, you have only three possible causes, so this is a
much more manageable list. Although there may be other possible causes that you can think of
(and it’s great that you can do that), for this example and in the interest of simplicity, only these
three are listed.
Here’s where it gets interesting. You now have to check each of these possibilities and fix
them if they are the cause of the problem.
Centralized Security in Remote Access Networks Summary
To have a complete security policy in place, authorization, authentication, and accounting
(AAA) must be implemented on a network. AAA not only allows full control over dial-up connections,
but login and exec access to devices. Tracking and auditing is accomplished through
the accounting services in AAA.
CiscoSecure is software that allows for centralized control over access to every device in your
network. It will run on Windows NT and Unix and provides RADIUS as well as TACACS+
authentication, authorization, and accounting services.
The two access modes, which are controlled by AAA, are character-mode and packet-mode
connections. Character-mode connections usually terminate at the access server or router, and
packet-mode connections are those that pass traffic through an access server or router.
Configuration of AAA services for Cisco devices has many facets. The administrator must
first configure how to authenticate users and then define which services those users will be
allowed to access. The optional accounting feature can be used to audit the user’s activity on
the system.
The use of a virtual template is a technology that enables the security server to supply the
access server with user-specific dialer profile information. Instead of each access server containing
user-specific dialer profile information, this information is kept on the security server and
downloaded to the access server when the user is authenticated.
(AAA) must be implemented on a network. AAA not only allows full control over dial-up connections,
but login and exec access to devices. Tracking and auditing is accomplished through
the accounting services in AAA.
CiscoSecure is software that allows for centralized control over access to every device in your
network. It will run on Windows NT and Unix and provides RADIUS as well as TACACS+
authentication, authorization, and accounting services.
The two access modes, which are controlled by AAA, are character-mode and packet-mode
connections. Character-mode connections usually terminate at the access server or router, and
packet-mode connections are those that pass traffic through an access server or router.
Configuration of AAA services for Cisco devices has many facets. The administrator must
first configure how to authenticate users and then define which services those users will be
allowed to access. The optional accounting feature can be used to audit the user’s activity on
the system.
The use of a virtual template is a technology that enables the security server to supply the
access server with user-specific dialer profile information. Instead of each access server containing
user-specific dialer profile information, this information is kept on the security server and
downloaded to the access server when the user is authenticated.
Troubleshooting Methodology
THE CCNP EXAM TOPICS COVERED IN THIS
CHAPTER INCLUDE THE FOLLOWING:
Know troubleshooting methodologies.
Troubleshooting is a skill that takes time and experience to fully
develop. To be successful when diagnosing and repairing network
failures, a good set of troubleshooting tools and skills is essential.
The information presented here is the foundation for the rest of the information covered on the
exam. This chapter emphasizes the importance of following a specific set of troubleshooting steps
when you try to diagnose and solve network problems. An effective troubleshooting methodology
is needed because of the complexity of today’s network environments. As a Cisco Certified Network
Professional (CCNP), you need to understand and know how to apply an efficient and systematic
troubleshooting methodology. Otherwise, you would be required to have a very intimate understanding
of the network you are troubleshooting. It is imperative that you learn troubleshooting
skills and understand the information available to you while solving network problems.
CHAPTER INCLUDE THE FOLLOWING:
Know troubleshooting methodologies.
Troubleshooting is a skill that takes time and experience to fully
develop. To be successful when diagnosing and repairing network
failures, a good set of troubleshooting tools and skills is essential.
The information presented here is the foundation for the rest of the information covered on the
exam. This chapter emphasizes the importance of following a specific set of troubleshooting steps
when you try to diagnose and solve network problems. An effective troubleshooting methodology
is needed because of the complexity of today’s network environments. As a Cisco Certified Network
Professional (CCNP), you need to understand and know how to apply an efficient and systematic
troubleshooting methodology. Otherwise, you would be required to have a very intimate understanding
of the network you are troubleshooting. It is imperative that you learn troubleshooting
skills and understand the information available to you while solving network problems.
AAA Accounting Commands
Command Description
aaa accounting command
level method server
Audits all commands at a specified level by using the specified
method. (The options are start-stop, stop-only, and wait-start.) Sends
this information to the server type (TACACS+ or RADIUS) specified.
aaa accounting
connection method
server
Audits all outbound connections (including Telnet and rlogin) to the
specified server type by using the specified method.
aaa accounting exec
method server
Audits the EXEC process with the specified method to the specified
server type.
aaa accounting network
method server
Audits network service requests (including SLIP, PPP, and ARAP
requests) to the specified server type by using the specified method.
aaa accounting system
method server
Audits system-level events by using the specified method to the
specified server type. This includes reload, for example. Because
a router reload is one of the ultimate denial-of-service attacks, it
would be useful to know what user identification was used to issue
the command.
aaa accounting
function start-stop
server
Documents the start and stop of a particular type of session specified
by the function parameter to the specified server type. Audit information
is sent in the background, negating any delay for the user.
aaa accounting
function stop-only
server
Sends a stop accounting notice at the end of a user process specified
by the function parameter to the specified server type.
aaa accounting
function wait-start
server
Similar to aaa accounting start-stop, this command documents
the start of a particular type of session specified by the function
parameter to the specified server type. However, the user is not permitted
to continue until the accounting server acknowledges the log
entry. This can delay user access.
aaa accounting
function method
{tacacs+ | radius}
Enables accounting information to be sent to the TACACS+ or
RADIUS accounting server for the specified function by using the
specified method.
NOTES:
One area in which accounting transcends security is charge-back. If accurate start
and stop times are recorded, a company could charge users for their time on the
system to offset the cost. Internet service providers (ISPs) have long considered
this as an alternative to the flat-rate model currently found in the United States.
aaa accounting command
level method server
Audits all commands at a specified level by using the specified
method. (The options are start-stop, stop-only, and wait-start.) Sends
this information to the server type (TACACS+ or RADIUS) specified.
aaa accounting
connection method
server
Audits all outbound connections (including Telnet and rlogin) to the
specified server type by using the specified method.
aaa accounting exec
method server
Audits the EXEC process with the specified method to the specified
server type.
aaa accounting network
method server
Audits network service requests (including SLIP, PPP, and ARAP
requests) to the specified server type by using the specified method.
aaa accounting system
method server
Audits system-level events by using the specified method to the
specified server type. This includes reload, for example. Because
a router reload is one of the ultimate denial-of-service attacks, it
would be useful to know what user identification was used to issue
the command.
aaa accounting
function start-stop
server
Documents the start and stop of a particular type of session specified
by the function parameter to the specified server type. Audit information
is sent in the background, negating any delay for the user.
aaa accounting
function stop-only
server
Sends a stop accounting notice at the end of a user process specified
by the function parameter to the specified server type.
aaa accounting
function wait-start
server
Similar to aaa accounting start-stop, this command documents
the start of a particular type of session specified by the function
parameter to the specified server type. However, the user is not permitted
to continue until the accounting server acknowledges the log
entry. This can delay user access.
aaa accounting
function method
{tacacs+ | radius}
Enables accounting information to be sent to the TACACS+ or
RADIUS accounting server for the specified function by using the
specified method.
NOTES:
One area in which accounting transcends security is charge-back. If accurate start
and stop times are recorded, a company could charge users for their time on the
system to offset the cost. Internet service providers (ISPs) have long considered
this as an alternative to the flat-rate model currently found in the United States.
Accounting Configuration
The accounting function records who did what and for how long. Because of this, it relies
upon the authentication process to provide part of the audit trail. For this reason, it is recommended
that accounts be established with easily identified usernames—typically a lastname,
first-initial configuration. This information is coupled with six accounting types, as
described in Table 32.5.
AAA Accounting Types
Accounting Type Function
Command Documents the commands submitted by the user and the privilege
level associated with them.
Connection Provides auditing of all outbound connections.
EXEC Logs user EXEC terminal sessions.
Network Audits all PPP, SLIP, and ARAP session traffic counts, including number
of packets and total bytes.
System Records system-level events.
Resource Provides information regarding connections that have failed, enabling
the administrator to evaluate user attempts.
The configuration of accounting is fairly simple, but there are a few choices that should be
considered. Table 32.6 provides a subset of the more common commands. Administrators
will need to balance the desire to obtain complete accounting records against the overhead
incurred. In Table 32.6, there is a function that is being accounted for that includes commands,
connections, system events, and so on. There is a method used to account for those
functions that includes start-stop, stop-only, and wait-start, and the server type to send this
information to.
upon the authentication process to provide part of the audit trail. For this reason, it is recommended
that accounts be established with easily identified usernames—typically a lastname,
first-initial configuration. This information is coupled with six accounting types, as
described in Table 32.5.
AAA Accounting Types
Accounting Type Function
Command Documents the commands submitted by the user and the privilege
level associated with them.
Connection Provides auditing of all outbound connections.
EXEC Logs user EXEC terminal sessions.
Network Audits all PPP, SLIP, and ARAP session traffic counts, including number
of packets and total bytes.
System Records system-level events.
Resource Provides information regarding connections that have failed, enabling
the administrator to evaluate user attempts.
The configuration of accounting is fairly simple, but there are a few choices that should be
considered. Table 32.6 provides a subset of the more common commands. Administrators
will need to balance the desire to obtain complete accounting records against the overhead
incurred. In Table 32.6, there is a function that is being accounted for that includes commands,
connections, system events, and so on. There is a method used to account for those
functions that includes start-stop, stop-only, and wait-start, and the server type to send this
information to.
Authorization Commands
Recall that authorization is the AAA process responsible for granting permission to access particular
components in the network. The administrator will need to define these permissions
based on corporate policy and user privileges. It is important to note that although a TACACS+
file was included in the previous section to illustrate authentication, the actual authorization
controls were not included.
The commands associated with authorization include parameters for the protocols that are
to be used and the method used for authorization. These commands are used after the authentication
phase of AAA, and they are described in Table 32.4.
TABLE 3 2 . 4 AAA Authorization Commands
Command Description
aaa authorization
network method
Performs authorization security on all network services—including
SLIP, PPP, and ARAP—using the method specified by the method
parameter. The method could be TACACS+, RADIUS, local, and so on.
aaa authorization
exec method
Authorizes the EXEC process with the specified AAA method.
aaa authorization
commands level 15
method
Authorizes all EXEC commands used at the specified level (0–15) by using
the specified method. In this example, this is level 15, which is regarded
as full authorization and normally associated with enable mode.
aaa authorization
config-commands
Uses AAA authorization for configuration mode commands.
aaa authorization
reverse-access
method
Uses AAA authorization specified by the method parameter for reverse
Telnet connections.
aaa authorization
function ifauthenticated
Permits the user to use the requested function only if the user is
authenticated.
aaa authorization
function local
Uses the local database for authorization for the specified function.
This database is stored on the router’s configuration in NVRAM.
aaa authorization
function radius
Uses RADIUS for authorization of the specified function.
aaa authorization
function tacacs+
Uses TACACS+ for authorization of the specified function.
TABLE 3 2 . 4 AAA Authorization Commands
Command Description
aaa authorization
network method
Performs authorization security on all network services—including
SLIP, PPP, and ARAP—using the method specified by the method
parameter. The method could be TACACS+, RADIUS, local, and so on.
aaa authorization
exec method
Authorizes the EXEC process with the specified AAA method.
aaa authorization
commands level 15
method
Authorizes all EXEC commands used at the specified level (0–15) by using
the specified method. In this example, this is level 15, which is regarded
as full authorization and normally associated with enable mode.
aaa authorization
config-commands
Uses AAA authorization for configuration mode commands.
aaa authorization
reverse-access
method
Uses AAA authorization specified by the method parameter for reverse
Telnet connections.
aaa authorization
function ifauthenticated
Permits the user to use the requested function only if the user is
authenticated.
aaa authorization
function local
Uses the local database for authorization for the specified function.
This database is stored on the router’s configuration in NVRAM.
aaa authorization
function radius
Uses RADIUS for authorization of the specified function.
aaa authorization
function tacacs+
Uses TACACS+ for authorization of the specified function.
components in the network. The administrator will need to define these permissions
based on corporate policy and user privileges. It is important to note that although a TACACS+
file was included in the previous section to illustrate authentication, the actual authorization
controls were not included.
The commands associated with authorization include parameters for the protocols that are
to be used and the method used for authorization. These commands are used after the authentication
phase of AAA, and they are described in Table 32.4.
TABLE 3 2 . 4 AAA Authorization Commands
Command Description
aaa authorization
network method
Performs authorization security on all network services—including
SLIP, PPP, and ARAP—using the method specified by the method
parameter. The method could be TACACS+, RADIUS, local, and so on.
aaa authorization
exec method
Authorizes the EXEC process with the specified AAA method.
aaa authorization
commands level 15
method
Authorizes all EXEC commands used at the specified level (0–15) by using
the specified method. In this example, this is level 15, which is regarded
as full authorization and normally associated with enable mode.
aaa authorization
config-commands
Uses AAA authorization for configuration mode commands.
aaa authorization
reverse-access
method
Uses AAA authorization specified by the method parameter for reverse
Telnet connections.
aaa authorization
function ifauthenticated
Permits the user to use the requested function only if the user is
authenticated.
aaa authorization
function local
Uses the local database for authorization for the specified function.
This database is stored on the router’s configuration in NVRAM.
aaa authorization
function radius
Uses RADIUS for authorization of the specified function.
aaa authorization
function tacacs+
Uses TACACS+ for authorization of the specified function.
TABLE 3 2 . 4 AAA Authorization Commands
Command Description
aaa authorization
network method
Performs authorization security on all network services—including
SLIP, PPP, and ARAP—using the method specified by the method
parameter. The method could be TACACS+, RADIUS, local, and so on.
aaa authorization
exec method
Authorizes the EXEC process with the specified AAA method.
aaa authorization
commands level 15
method
Authorizes all EXEC commands used at the specified level (0–15) by using
the specified method. In this example, this is level 15, which is regarded
as full authorization and normally associated with enable mode.
aaa authorization
config-commands
Uses AAA authorization for configuration mode commands.
aaa authorization
reverse-access
method
Uses AAA authorization specified by the method parameter for reverse
Telnet connections.
aaa authorization
function ifauthenticated
Permits the user to use the requested function only if the user is
authenticated.
aaa authorization
function local
Uses the local database for authorization for the specified function.
This database is stored on the router’s configuration in NVRAM.
aaa authorization
function radius
Uses RADIUS for authorization of the specified function.
aaa authorization
function tacacs+
Uses TACACS+ for authorization of the specified function.
Authorization Configuration
Authorization defines the network services that are available to an individual or group. It provides
an easy means of allowing privileged-mode (enable-mode) access while restricting the
commands that can be executed. For example, you might want to isolate most enable commands
to a single administrator or manager, while allowing operators to perform limited diagnostic
functions. More experienced operators would be granted higher levels of authorization—
for example, they might be permitted to shut down an interface. The unrestricted enable-mode
administrator would be required for additional functions.
Use care in restricting administrative rights to the router. Although this is a
helpful option when allocating rights to vendors and other parties, too restrictive
a policy will lead to the distribution of the unrestricted account information,
which can create a larger security risk.
an easy means of allowing privileged-mode (enable-mode) access while restricting the
commands that can be executed. For example, you might want to isolate most enable commands
to a single administrator or manager, while allowing operators to perform limited diagnostic
functions. More experienced operators would be granted higher levels of authorization—
for example, they might be permitted to shut down an interface. The unrestricted enable-mode
administrator would be required for additional functions.
Use care in restricting administrative rights to the router. Although this is a
helpful option when allocating rights to vendors and other parties, too restrictive
a policy will lead to the distribution of the unrestricted account information,
which can create a larger security risk.
Catalyst Switch Configuration
On the Cisco Catalyst series switch platform running Catalyst Operating System (CatOS), the
authentication commands present themselves differently, but the resulting behavior is the same.
The following configuration, like the router configuration, uses TACACS+ for login and enable
(privileged) mode:
#tacacs+
set tacacs server 10.1.98.36 primary
set tacacs server 10.1.5.36
set tacacs attempts 3
set tacacs directedrequest disable
set tacacs key tjelkprp
set tacacs timeout 5
set authentication login tacacs enable
set authentication login local enable
set authentication enable tacacs enable
set authentication enable local enable
Again, this configuration file is an excerpt from the Catalyst switch configuration file—
displayed with the show config command. There are two TACACS+ servers defined; however,
notice that one is defined as primary. On the router, the first server listed is defaulted
to primary, but the switch allows for the primary’s configuration by using the primary keyword.
Don’t be too concerned with understanding the switch configuration—the test
focuses only on the router-based commands. The configuration is provided here so readers
who have not previously experienced Catalyst commands can become familiar with them.
The remainder of this chapter focuses only on the router commands.
The switch commands in this chapter are based on version 4.5.5 of the Catalyst
code. There might be minor differences with other versions. show config or
write terminal are often used to show the configuration information.
authentication commands present themselves differently, but the resulting behavior is the same.
The following configuration, like the router configuration, uses TACACS+ for login and enable
(privileged) mode:
#tacacs+
set tacacs server 10.1.98.36 primary
set tacacs server 10.1.5.36
set tacacs attempts 3
set tacacs directedrequest disable
set tacacs key tjelkprp
set tacacs timeout 5
set authentication login tacacs enable
set authentication login local enable
set authentication enable tacacs enable
set authentication enable local enable
Again, this configuration file is an excerpt from the Catalyst switch configuration file—
displayed with the show config command. There are two TACACS+ servers defined; however,
notice that one is defined as primary. On the router, the first server listed is defaulted
to primary, but the switch allows for the primary’s configuration by using the primary keyword.
Don’t be too concerned with understanding the switch configuration—the test
focuses only on the router-based commands. The configuration is provided here so readers
who have not previously experienced Catalyst commands can become familiar with them.
The remainder of this chapter focuses only on the router commands.
The switch commands in this chapter are based on version 4.5.5 of the Catalyst
code. There might be minor differences with other versions. show config or
write terminal are often used to show the configuration information.
A Sample TACACS+ Configuration File
The easiest way to understand the authorization function is to examine a configuration file that
controls authorized services. Look at the following sample configuration file that controls
authorized services:
#TACACS+ V2.1 configuration file
#created 5/14/03
#edited 8/26/03
#
#If user doesn't appear in the config file user/etc/password
default authentication = file /etc/passwd
accounting file = /home1/logs/tacacs+.accounting
#Must be same as router IOS "tacacs-server key"
key = tjelkprp
#
user=netops {
member=operator
login=cleartext dilbert
}
user=rpadjen {
# Robert Padjen
default service=permit
login=cleartext yummy
}
group=operator {
name="Network Operator"
cmd=debug {
permit .*
}
cmd=write {
permit terminal
}
cmd=clear {
permit .*
}
cmd=show {
#permit show commands
permit .*
}
}
user=shayna {
# Shayna Padjen
member=operator_plus
login=cleartext flatshoe
}
group=operator_plus {
name="Network Operator Plus"
cmd=debug {
permit .*
}
cmd=write {
permit terminal
}
cmd=clear {
permit .*
}
#permit show commands
cmd=show {
permit .*
}
cmd=configure {
permit terminal
}
cmd=interface {
permit .*
}
cmd=shutdown {
permit .*
}
cmd=no {
permit shutdown
}
}
This file establishes a number of user accounts and authorization rights. The first group,
operator, is provided with basic diagnostic and administrative functions, while the operator_
plus group is enhanced with shutdown, interface, and configure commands. All commands
are available to one administrator. Note that Shayna is a member of operator_plus,
and Rob is allowed full access.
Pay particular attention to a few additional items about this specific configuration file. First,
the passwords are in cleartext, meaning that anyone with access to the server can obtain them.
Most configuration files are encrypted. Second, observe that restrictions can be quite granular
and could include functions such as Ping while blocking extended ping.
Please refer to the documentation that accompanies your server for syntax and
configuration instructions specific to your installation.
controls authorized services. Look at the following sample configuration file that controls
authorized services:
#TACACS+ V2.1 configuration file
#created 5/14/03
#edited 8/26/03
#
#If user doesn't appear in the config file user/etc/password
default authentication = file /etc/passwd
accounting file = /home1/logs/tacacs+.accounting
#Must be same as router IOS "tacacs-server key"
key = tjelkprp
#
user=netops {
member=operator
login=cleartext dilbert
}
user=rpadjen {
# Robert Padjen
default service=permit
login=cleartext yummy
}
group=operator {
name="Network Operator"
cmd=debug {
permit .*
}
cmd=write {
permit terminal
}
cmd=clear {
permit .*
}
cmd=show {
#permit show commands
permit .*
}
}
user=shayna {
# Shayna Padjen
member=operator_plus
login=cleartext flatshoe
}
group=operator_plus {
name="Network Operator Plus"
cmd=debug {
permit .*
}
cmd=write {
permit terminal
}
cmd=clear {
permit .*
}
#permit show commands
cmd=show {
permit .*
}
cmd=configure {
permit terminal
}
cmd=interface {
permit .*
}
cmd=shutdown {
permit .*
}
cmd=no {
permit shutdown
}
}
This file establishes a number of user accounts and authorization rights. The first group,
operator, is provided with basic diagnostic and administrative functions, while the operator_
plus group is enhanced with shutdown, interface, and configure commands. All commands
are available to one administrator. Note that Shayna is a member of operator_plus,
and Rob is allowed full access.
Pay particular attention to a few additional items about this specific configuration file. First,
the passwords are in cleartext, meaning that anyone with access to the server can obtain them.
Most configuration files are encrypted. Second, observe that restrictions can be quite granular
and could include functions such as Ping while blocking extended ping.
Please refer to the documentation that accompanies your server for syntax and
configuration instructions specific to your installation.
Router Access Modes
Router Access Modes
A Cisco router can be accessed by using one of two access modes. These are broadly categorized
as character mode and packet mode. In essence, the difference between these modes can be best
understood by looking at the commands that configure character and packet modes. You
should understand the difference in the modes and use this section as an introduction to the configuration
command syntax.
Character-Mode Connections
Character-mode connections
describe character-based access, including access via the VTY,
TTY, AUX (auxiliary), and CON (console) ports. Although such access might be through a
packet-based network—Telnet, for example—the connection is still viewed as being character
based. The AAA commands that configure character-mode access are as follows:
login
exec
nasi
connection
arap
enable
command
Character-mode access usually includes connections only to the router or network device.
Table 32.1 includes explanations of these commands.
TABLE 3 2 . 1
Character-Mode Authentication and Authorization Commands
Command Description
aaa authentication enable
default tacacs+ enable
Uses TACACS+ to determine whether the user can access
enabled mode. If TACACS+ is unavailable, the local enable
password will be used.
aaa authorization exec
tacacs+ local
Determines whether the user is allowed access to the EXEC
shell. This example provides for TACACS+ authentication, and
should TACACS+ fail, it permits authorization via the local
database. The local database is populated with the
username
command.
aaa authorization command
n
tacacs+ local
Runs authorization for all commands at privilege level
n
(a
number between 0 and 15). Every line entered by a user can be
controlled and authorized by TACACS+, although performance
can suffer.
username
user
password
password
Creates or adds to the local database with a username of
user
and the password of password. This database is stored in the
router’s configuration file in NVRAM (nonvolatile random
access memory), and it can be accessed upon authentication
failure depending on configuration.
A Cisco router can be accessed by using one of two access modes. These are broadly categorized
as character mode and packet mode. In essence, the difference between these modes can be best
understood by looking at the commands that configure character and packet modes. You
should understand the difference in the modes and use this section as an introduction to the configuration
command syntax.
Character-Mode Connections
Character-mode connections
describe character-based access, including access via the VTY,
TTY, AUX (auxiliary), and CON (console) ports. Although such access might be through a
packet-based network—Telnet, for example—the connection is still viewed as being character
based. The AAA commands that configure character-mode access are as follows:
login
exec
nasi
connection
arap
enable
command
Character-mode access usually includes connections only to the router or network device.
Table 32.1 includes explanations of these commands.
TABLE 3 2 . 1
Character-Mode Authentication and Authorization Commands
Command Description
aaa authentication enable
default tacacs+ enable
Uses TACACS+ to determine whether the user can access
enabled mode. If TACACS+ is unavailable, the local enable
password will be used.
aaa authorization exec
tacacs+ local
Determines whether the user is allowed access to the EXEC
shell. This example provides for TACACS+ authentication, and
should TACACS+ fail, it permits authorization via the local
database. The local database is populated with the
username
command.
aaa authorization command
n
tacacs+ local
Runs authorization for all commands at privilege level
n
(a
number between 0 and 15). Every line entered by a user can be
controlled and authorized by TACACS+, although performance
can suffer.
username
user
password
password
Creates or adds to the local database with a username of
user
and the password of password. This database is stored in the
router’s configuration file in NVRAM (nonvolatile random
access memory), and it can be accessed upon authentication
failure depending on configuration.
Router Configuration
The following is extracted from the full configuration file of the router to highlight the commands
used for AAA configuration:
aaa new-model
aaa authentication login default tacacs+ enable
aaa authentication enable default tacacs+ enable
aaa accounting exec start-stop tacacs+
tacacs-server host 10.1.98.36
tacacs-server host 10.1.5.36
tacacs-server key tjelkprp
The preceding output is an example of a typical router configuration. This output starts the
AAA service, establishes authentication services for both the login and enable processes, and
audits the start and end times of each access. The two TACACS+ servers noted here are defined,
and the preshared key is assigned.
In this example (which uses TACACS+), the aaa authentication command is used to
define the type of authentication protocol. The enable keyword at the end of the two authentication
commands allows the local enable secret password (use of the enable password would
be used if the secret is not defined, but this is not recommended from a security perspective)
to be used if network connectivity is lost between the security server and router; however, this
also can be considered a security risk. This risk is minor, considering that the attacker would
have to physically access the router or compromise the internal network sufficiently to change
routes or block packets. Here, the tacacs-server command is being used to define the IP
address of each TACACS+ server. In this example, the server key is being used to provide basic
security over the communications link to the security server. Note that this configuration
includes an aaa accounting command, which instructs the router to log the start and stop
times of an exec session to the TACACS+ server.
Each of these commands is documented at the end of this chapter.
used for AAA configuration:
aaa new-model
aaa authentication login default tacacs+ enable
aaa authentication enable default tacacs+ enable
aaa accounting exec start-stop tacacs+
tacacs-server host 10.1.98.36
tacacs-server host 10.1.5.36
tacacs-server key tjelkprp
The preceding output is an example of a typical router configuration. This output starts the
AAA service, establishes authentication services for both the login and enable processes, and
audits the start and end times of each access. The two TACACS+ servers noted here are defined,
and the preshared key is assigned.
In this example (which uses TACACS+), the aaa authentication command is used to
define the type of authentication protocol. The enable keyword at the end of the two authentication
commands allows the local enable secret password (use of the enable password would
be used if the secret is not defined, but this is not recommended from a security perspective)
to be used if network connectivity is lost between the security server and router; however, this
also can be considered a security risk. This risk is minor, considering that the attacker would
have to physically access the router or compromise the internal network sufficiently to change
routes or block packets. Here, the tacacs-server command is being used to define the IP
address of each TACACS+ server. In this example, the server key is being used to provide basic
security over the communications link to the security server. Note that this configuration
includes an aaa accounting command, which instructs the router to log the start and stop
times of an exec session to the TACACS+ server.
Each of these commands is documented at the end of this chapter.
AAA Configuration
Although AAA was designed to centralize access control, it still requires configuration on each
and every network device. Fortunately, after AAA is configured, there are few instances when
the administrator will need to alter its configuration—for example, when the encryption key is
changed. Aside from such minor alterations, all changes—including those for user accounts—
are invoked at the security server. This configuration process lets the router or access device
know about the type of security to be used, the location of the security server, and the passwords
or other information needed to facilitate communications.
In addition to these configuration commands, the administrator must establish
network-level connectivity between the access device and the security server.
This might require access list modification or route entries.
Table 32.3 outlines some of the AAA commands, including those for authentication and
accounting. The configurations that relate to these commands are shown later in this section.
TABLE 3 2 . 3 Overview of AAA Commands and Configuration
Command Description
aaa new-model Enables AAA services on the router. new-model reflects changes from
the initial implementation, which is no longer supported. In the
absence of other AAA commands, the local database will be used for
username and password. If no database is present and no other AAA
method is specified, this command will lock out the router.
aaa authentication
login default tacacs+
enable
Configures TACACS+ to be the default method used for login-level
access. If TACACS+ is unavailable, use the local enable password.
aaa authentication
enable default
tacacs+ enable
Configures TACACS+ to be the default method used for enable-level
access. If TACACS+ is unavailable, use the local enable password.
aaa accounting exec
start-stop tacacs+
Configures the accounting process, logging the start and stop times
of each exec session access.
tacacs-server host
10.1.98.36
Specifies the IP address of the TACACS+ server. The singleconnection
parameter can be used to improve performance by
maintaining a single TCP session as opposed to starting a separate
session for each authentication.
tacacs-server key
tjelkprp
Specifies the encryption key to be used for communications between
the router and TACACS+ server.
and every network device. Fortunately, after AAA is configured, there are few instances when
the administrator will need to alter its configuration—for example, when the encryption key is
changed. Aside from such minor alterations, all changes—including those for user accounts—
are invoked at the security server. This configuration process lets the router or access device
know about the type of security to be used, the location of the security server, and the passwords
or other information needed to facilitate communications.
In addition to these configuration commands, the administrator must establish
network-level connectivity between the access device and the security server.
This might require access list modification or route entries.
Table 32.3 outlines some of the AAA commands, including those for authentication and
accounting. The configurations that relate to these commands are shown later in this section.
TABLE 3 2 . 3 Overview of AAA Commands and Configuration
Command Description
aaa new-model Enables AAA services on the router. new-model reflects changes from
the initial implementation, which is no longer supported. In the
absence of other AAA commands, the local database will be used for
username and password. If no database is present and no other AAA
method is specified, this command will lock out the router.
aaa authentication
login default tacacs+
enable
Configures TACACS+ to be the default method used for login-level
access. If TACACS+ is unavailable, use the local enable password.
aaa authentication
enable default
tacacs+ enable
Configures TACACS+ to be the default method used for enable-level
access. If TACACS+ is unavailable, use the local enable password.
aaa accounting exec
start-stop tacacs+
Configures the accounting process, logging the start and stop times
of each exec session access.
tacacs-server host
10.1.98.36
Specifies the IP address of the TACACS+ server. The singleconnection
parameter can be used to improve performance by
maintaining a single TCP session as opposed to starting a separate
session for each authentication.
tacacs-server key
tjelkprp
Specifies the encryption key to be used for communications between
the router and TACACS+ server.
Packet-Mode Connections
Packet-mode connections
include most dial-up connections, including the following:
async
group-async
serial
ISDN BRI
ISDN PRI
Packet-mode connections typically secure connections that pass traffic through the network
device. You use the ppp, network, and arap AAA commands to control packet-mode connections.
Table 32.2 offers a list with explanations of these commands.
These sections do not provide a complete breakdown of all possible commands,
but instead they introduce the more common commands. Please refer
to the documentation specific to your version of the IOS for a current listing of
all commands and options or use the incorporated Help function.
TABLE 3 2 . 2 Packet-Mode Authentication Commands
Command Description
aaa authentication ppp
user if-needed tacacs+
AAA is used for PPP packet-mode challenges. The list user is
used first, and if unsuccessful, TACACS+ will be used.
aaa authorization network
tacacs+ if-authenticated
TACACS+ is used to determine whether the user is permitted to
make packet-mode connections if the user is authenticated.
interface async16 ppp
authentication chap user
This is a new command for this chapter in that it associates
an AAA function with an interface. Specifically, line async16 is
instructed to use the list user for CHAP authentication. Note that
an AAA server (RADIUS, and so on) is not used.
include most dial-up connections, including the following:
async
group-async
serial
ISDN BRI
ISDN PRI
Packet-mode connections typically secure connections that pass traffic through the network
device. You use the ppp, network, and arap AAA commands to control packet-mode connections.
Table 32.2 offers a list with explanations of these commands.
These sections do not provide a complete breakdown of all possible commands,
but instead they introduce the more common commands. Please refer
to the documentation specific to your version of the IOS for a current listing of
all commands and options or use the incorporated Help function.
TABLE 3 2 . 2 Packet-Mode Authentication Commands
Command Description
aaa authentication ppp
user if-needed tacacs+
AAA is used for PPP packet-mode challenges. The list user is
used first, and if unsuccessful, TACACS+ will be used.
aaa authorization network
tacacs+ if-authenticated
TACACS+ is used to determine whether the user is permitted to
make packet-mode connections if the user is authenticated.
interface async16 ppp
authentication chap user
This is a new command for this chapter in that it associates
an AAA function with an interface. Specifically, line async16 is
instructed to use the list user for CHAP authentication. Note that
an AAA server (RADIUS, and so on) is not used.
How AAA Works
How AAA Works
It is important to remember that AAA is simply a grouping of three security functions—authentication,
authorization, and accounting. Most texts examine each component as an isolated process,
and although this is perhaps more accurate, here they have been placed into a three-step
process to better communicate the interactions between each service. For example, it is perfectly
valid to use only authentication and authorization while omitting accounting, but if you do so,
administrators will lose the auditing benefits that are provided by the auditing service.
Step 1: Authentication
Authentication
is the first facet of the three security elements, and it provides a basis for the remaining
two components. Authentication provides the “who” in the AAA model. Like journalists who
ask themselves the questions they must answer to make their story good (Who?, What?, Where?,
When?, and How?), administrators need to ask who is involved in their system; it is one of the fundamental
pieces of information they need to set up their system. Unfortunately, in computing, as in
non-computing situations, it can be fairly simple to lie about one’s identity.
To facilitate the authentication process, most systems require both a username and a
password—it is hoped the password will be maintained in confidence in order to preclude
the potential of compromise. By requiring two elements of identity, the computer-based
system doubles the likelihood that the user is accurately identified.
However, it is possible to obtain, lie about, or guess both pieces of information. The likelihood
of accurate authentication is stronger if a physical element is added. In non-computing situations,
this might include a passport or driver’s license; in the computer world, it might include a tokenbased
device. As presented in the CiscoSecure section of this chapter, there are many products that
can provide this service as a software receiver of the physical code card data.
Step 2: Authorization
After the identity of the user has been established, a decision must be made regarding what
rights that user can exercise. This is called
authorization
, and is assigned by the administrator
based on the requirements and business policies of the organization. An example of authorization
would include permissions to access a remote access device or the ability to print a file.
Because authentication and authorization are so involved and dependent on each other, they are
regarded as a single security component in most environments.
Step 3: Accounting
Whereas authentication and authorization work to prevent unauthorized access,
accounting
provides a means of verifying that only authorized users obtain access. In addition, accounting
is used to audit the actions of an authorized user.
An accounting record relies on the authenticity of the authentication process—a fraudulent
user might provide a valid login, but the accounting feature provides the audit trail required to
assess the damage. This log provides a record of when an activity occurred and what action was
performed—connecting to a router, for example.
It is important to remember that AAA is simply a grouping of three security functions—authentication,
authorization, and accounting. Most texts examine each component as an isolated process,
and although this is perhaps more accurate, here they have been placed into a three-step
process to better communicate the interactions between each service. For example, it is perfectly
valid to use only authentication and authorization while omitting accounting, but if you do so,
administrators will lose the auditing benefits that are provided by the auditing service.
Step 1: Authentication
Authentication
is the first facet of the three security elements, and it provides a basis for the remaining
two components. Authentication provides the “who” in the AAA model. Like journalists who
ask themselves the questions they must answer to make their story good (Who?, What?, Where?,
When?, and How?), administrators need to ask who is involved in their system; it is one of the fundamental
pieces of information they need to set up their system. Unfortunately, in computing, as in
non-computing situations, it can be fairly simple to lie about one’s identity.
To facilitate the authentication process, most systems require both a username and a
password—it is hoped the password will be maintained in confidence in order to preclude
the potential of compromise. By requiring two elements of identity, the computer-based
system doubles the likelihood that the user is accurately identified.
However, it is possible to obtain, lie about, or guess both pieces of information. The likelihood
of accurate authentication is stronger if a physical element is added. In non-computing situations,
this might include a passport or driver’s license; in the computer world, it might include a tokenbased
device. As presented in the CiscoSecure section of this chapter, there are many products that
can provide this service as a software receiver of the physical code card data.
Step 2: Authorization
After the identity of the user has been established, a decision must be made regarding what
rights that user can exercise. This is called
authorization
, and is assigned by the administrator
based on the requirements and business policies of the organization. An example of authorization
would include permissions to access a remote access device or the ability to print a file.
Because authentication and authorization are so involved and dependent on each other, they are
regarded as a single security component in most environments.
Step 3: Accounting
Whereas authentication and authorization work to prevent unauthorized access,
accounting
provides a means of verifying that only authorized users obtain access. In addition, accounting
is used to audit the actions of an authorized user.
An accounting record relies on the authenticity of the authentication process—a fraudulent
user might provide a valid login, but the accounting feature provides the audit trail required to
assess the damage. This log provides a record of when an activity occurred and what action was
performed—connecting to a router, for example.
CiscoSecure
The CiscoSecure product is Cisco’s security server solution. This product incorporates many
services, including TACACS
+
and RADIUS servers, as well as logging functionality.
CiscoSecure uses web-based interfaces and Java to provide multiple administrators with
access to the server. Though the product supports both Internet Explorer and Netscape, it ships
with a Netscape FastTrack Server, and some administrators find it to be more reliable with the
Netscape client. CiscoSecure also relies on a relational database to manage accounts and store
information—currently it supports the Oracle and Sybase database platforms.
For enhanced security, administrators can choose to use
one-time challenge tokens
. These tokens
provide for the use of a different password for each login—a tactic that prevents session replay and
other techniques that would otherwise compromise security. Token cards from CRYPTOCard,
Enigma Logic, and Security Dynamics Technologies are supported with CiscoSecure.
services, including TACACS
+
and RADIUS servers, as well as logging functionality.
CiscoSecure uses web-based interfaces and Java to provide multiple administrators with
access to the server. Though the product supports both Internet Explorer and Netscape, it ships
with a Netscape FastTrack Server, and some administrators find it to be more reliable with the
Netscape client. CiscoSecure also relies on a relational database to manage accounts and store
information—currently it supports the Oracle and Sybase database platforms.
For enhanced security, administrators can choose to use
one-time challenge tokens
. These tokens
provide for the use of a different password for each login—a tactic that prevents session replay and
other techniques that would otherwise compromise security. Token cards from CRYPTOCard,
Enigma Logic, and Security Dynamics Technologies are supported with CiscoSecure.
Security Terminology
Security Terminology
Many of the terms presented in the other chapters of this book are familiar or easily interpreted
from the context. This chapter’s terms differ slightly because they might not be as familiar.
Treat this list as a high-level introduction to these security components, but realize that more
detail will be provided throughout the chapter:
Authentication
The
authentication
function answers the fundamental question: Who is the
user? By performing this function, you ensure that unwanted intruders will be denied access to
the network while other users will be permitted. The user’s identity can then be used to determine
access permissions and to provide an audit trail of activity.
Authorization
The
authorization
function often works in concert with authentication. It provides
a means for defining which network services will be available to the authenticated user.
Accounting
Accounting
is an optional function in AAA; however, it is responsible for the
auditing process, which can greatly enhance the security of the network. Accounting can also
log the activities of the user, including the time that they start and stop their connection.
RADIUS
Remote Access Dial-In User Service (RADIUS)
is a protocol that is used to communicate
between the remote access device and an authentication server. Sometimes an authentication
server running RADIUS will be called a
RADIUS
server
.
TACACS+
Enhanced Terminal Access Controller Access Control System (TACACS+
is a protocol
similar to RADIUS. Sometimes the server is called a
T-plus
or
T+
server
.
Security server
A
security server
runs the protocol—TACACS
+
or RADIUS—that is used to
provide AAA services. It should be secured and redundant, especially if it provides business-critical
access control. CiscoSecure is Cisco’s version of this type of server and is available on Windows NT
and Unix.
Many of the terms presented in the other chapters of this book are familiar or easily interpreted
from the context. This chapter’s terms differ slightly because they might not be as familiar.
Treat this list as a high-level introduction to these security components, but realize that more
detail will be provided throughout the chapter:
Authentication
The
authentication
function answers the fundamental question: Who is the
user? By performing this function, you ensure that unwanted intruders will be denied access to
the network while other users will be permitted. The user’s identity can then be used to determine
access permissions and to provide an audit trail of activity.
Authorization
The
authorization
function often works in concert with authentication. It provides
a means for defining which network services will be available to the authenticated user.
Accounting
Accounting
is an optional function in AAA; however, it is responsible for the
auditing process, which can greatly enhance the security of the network. Accounting can also
log the activities of the user, including the time that they start and stop their connection.
RADIUS
Remote Access Dial-In User Service (RADIUS)
is a protocol that is used to communicate
between the remote access device and an authentication server. Sometimes an authentication
server running RADIUS will be called a
RADIUS
server
.
TACACS+
Enhanced Terminal Access Controller Access Control System (TACACS+
is a protocol
similar to RADIUS. Sometimes the server is called a
T-plus
or
T+
server
.
Security server
A
security server
runs the protocol—TACACS
+
or RADIUS—that is used to
provide AAA services. It should be secured and redundant, especially if it provides business-critical
access control. CiscoSecure is Cisco’s version of this type of server and is available on Windows NT
and Unix.
CiscoSecure’s Response
CiscoSecure’s Response to Brute Force and Denial-of-Service Attacks
The CiscoSecure product, like other such products, has the capability to disable accounts automatically
in response to brute force attacks. This is accomplished by
intruder detection
, in which
the software assumes that the party is an intruder after a certain number of failed logins. A
brute
force attack
is one in which the attacker bombards the system with login attempts. Ultimately,
such an attack can lead to access—especially when passwords and account information are relatively
simple. By detecting such an attack, products can disable the account before it is compromised.
Frequently, such logic is limited to the number of attempts per unit of time, however. For
example, a brute force rule might allow five bad login attempts per hour before locking the
account for a day, or it might detect three bad passwords and then lock the account until the
administrator releases it.
Unfortunately, most solutions to a brute force attack lead to another type of attack: denialof-
service. A
denial-of-service attack
usually does not lead to the access of private information;
rather, as the name suggests, it prevents legitimate users from obtaining that data or
using the resource. Administrators must balance the impact of brute force compromises
against the potential of blocking access to legitimate users as a result of this protection. As
with most products, including CiscoSecure and others, the responsibility to balance access
control with access is placed on the administrator.
The CiscoSecure product, like other such products, has the capability to disable accounts automatically
in response to brute force attacks. This is accomplished by
intruder detection
, in which
the software assumes that the party is an intruder after a certain number of failed logins. A
brute
force attack
is one in which the attacker bombards the system with login attempts. Ultimately,
such an attack can lead to access—especially when passwords and account information are relatively
simple. By detecting such an attack, products can disable the account before it is compromised.
Frequently, such logic is limited to the number of attempts per unit of time, however. For
example, a brute force rule might allow five bad login attempts per hour before locking the
account for a day, or it might detect three bad passwords and then lock the account until the
administrator releases it.
Unfortunately, most solutions to a brute force attack lead to another type of attack: denialof-
service. A
denial-of-service attack
usually does not lead to the access of private information;
rather, as the name suggests, it prevents legitimate users from obtaining that data or
using the resource. Administrators must balance the impact of brute force compromises
against the potential of blocking access to legitimate users as a result of this protection. As
with most products, including CiscoSecure and others, the responsibility to balance access
control with access is placed on the administrator.
Centralized Security in Remote Access Networks
Centralized Security
in Remote Access
Networks
THE CCNP EXAM TOPICS COVERED IN THIS
CHAPTER INCLUDE THE FOLLOWING:
Know the security features of CiscoSecure and the
operation of a CiscoSecure server.
Understand the commands and procedures used to configure
routers to access a CiscoSecure server and to use AAA.
Know the commands used to configure AAA on a router to
control access from remote access clients.
Remote access encompasses two elements:
The communications channel between two points, or the
connection
Access control, or determining who or what can access the
network and its data
These concepts are known as
authentication, authorization, and accounting (AAA)
. AAA
is Cisco’s way of explaining the access control components and processes, and it is the topic
of this chapter.
This book has covered many of the fundamental elements of authentication and authorization—
particularly in the context of Challenge Handshake Authentication Protocol, or CHAP (see
Chapter 24, “Point-to-Point Protocol”). This chapter explores these concepts further, but the discussion
focuses more on the theoretical concepts of security and Cisco’s preferred implementation
of each of these concepts. AAA services are essential to providing centralized access control services,
which is a recurrent theme in this chapter and most Cisco security implementations
in Remote Access
Networks
THE CCNP EXAM TOPICS COVERED IN THIS
CHAPTER INCLUDE THE FOLLOWING:
Know the security features of CiscoSecure and the
operation of a CiscoSecure server.
Understand the commands and procedures used to configure
routers to access a CiscoSecure server and to use AAA.
Know the commands used to configure AAA on a router to
control access from remote access clients.
Remote access encompasses two elements:
The communications channel between two points, or the
connection
Access control, or determining who or what can access the
network and its data
These concepts are known as
authentication, authorization, and accounting (AAA)
. AAA
is Cisco’s way of explaining the access control components and processes, and it is the topic
of this chapter.
This book has covered many of the fundamental elements of authentication and authorization—
particularly in the context of Challenge Handshake Authentication Protocol, or CHAP (see
Chapter 24, “Point-to-Point Protocol”). This chapter explores these concepts further, but the discussion
focuses more on the theoretical concepts of security and Cisco’s preferred implementation
of each of these concepts. AAA services are essential to providing centralized access control services,
which is a recurrent theme in this chapter and most Cisco security implementations
Authentication, Authorization, and Accounting
Authentication, Authorization,
and Accounting
Regarded as distinct elements, authentication, authorization, and accounting (AAA) all work cooperatively
to establish and enforce a security model. This model is the result of a
security policy
, which
should define an overall set of standards that will be used by the organization to secure and protect
its assets. This policy can include definitions of access rights that will be assigned to different groups
and the protocols that will be used for various functions. For example, one policy statement might
include that TACACS
+
is the sole protocol used and that SSH, a secure tool used for administration,
is preferred over Telnet.
It is important to understand how authentication, authorization, and accounting work
together to promote and support a security model. In this chapter, you will learn about how
AAA works, as well as how AAA functions in Cisco’s router access modes. AAA services are
the basic tenet of Cisco remote access solutions, and, although their presentation has been left
to the end of the book, you should find that the Physical and Network layers supplement these
concepts well. This includes physical security, the use of access lists, static or authenticated IP
routing, and other security techniques.
and Accounting
Regarded as distinct elements, authentication, authorization, and accounting (AAA) all work cooperatively
to establish and enforce a security model. This model is the result of a
security policy
, which
should define an overall set of standards that will be used by the organization to secure and protect
its assets. This policy can include definitions of access rights that will be assigned to different groups
and the protocols that will be used for various functions. For example, one policy statement might
include that TACACS
+
is the sole protocol used and that SSH, a secure tool used for administration,
is preferred over Telnet.
It is important to understand how authentication, authorization, and accounting work
together to promote and support a security model. In this chapter, you will learn about how
AAA works, as well as how AAA functions in Cisco’s router access modes. AAA services are
the basic tenet of Cisco remote access solutions, and, although their presentation has been left
to the end of the book, you should find that the Physical and Network layers supplement these
concepts well. This includes physical security, the use of access lists, static or authenticated IP
routing, and other security techniques.
Network Address Translation and Port Address Translation Summary
As the Internet grows and companies need more and more IP addresses, the number of available
IP addresses diminishes. This is one of the main reasons for the implementation of NAT and
PAT—technologies that are critical to overcoming the shortage of IP addresses. You need to
understand how NAT and PAT operate and how to configure each of them.
These two protocols, which allow for specifically defined address translations, provide some
other interesting uses as well. For instance, NAT and PAT enable private IP networks to use
unregistered IP addresses to access outside networks such as the Internet. They also provide the
ability to reuse assigned IP addresses already in use on the Internet. In addition, they appropriately
translate the addresses in two merged intranets, such as those of two merged companies.
Finally, NAT and PAT translate internal IP addresses assigned by an old Internet service provider
(ISP) to a new ISP’s newly assigned addresses without manual configuration of all the local
network interfaces.
There are some disadvantages to using NAT and PAT in a network. Specifically, they don’t
allow for a full end-to-end communication between two hosts. Some protocols carry IP
address information in the payload of the packet that might not get translated by the border
NAT router.
There are many IOS commands used specifically for troubleshooting NAT problems. The
show ip nat translations command is one of the most useful, in addition to the debug ip
nat feature. PAT also has its own troubleshooting commands; the show ip pat command is
the most important.
IP addresses diminishes. This is one of the main reasons for the implementation of NAT and
PAT—technologies that are critical to overcoming the shortage of IP addresses. You need to
understand how NAT and PAT operate and how to configure each of them.
These two protocols, which allow for specifically defined address translations, provide some
other interesting uses as well. For instance, NAT and PAT enable private IP networks to use
unregistered IP addresses to access outside networks such as the Internet. They also provide the
ability to reuse assigned IP addresses already in use on the Internet. In addition, they appropriately
translate the addresses in two merged intranets, such as those of two merged companies.
Finally, NAT and PAT translate internal IP addresses assigned by an old Internet service provider
(ISP) to a new ISP’s newly assigned addresses without manual configuration of all the local
network interfaces.
There are some disadvantages to using NAT and PAT in a network. Specifically, they don’t
allow for a full end-to-end communication between two hosts. Some protocols carry IP
address information in the payload of the packet that might not get translated by the border
NAT router.
There are many IOS commands used specifically for troubleshooting NAT problems. The
show ip nat translations command is one of the most useful, in addition to the debug ip
nat feature. PAT also has its own troubleshooting commands; the show ip pat command is
the most important.
Cisco Access Control Solutions
Consider your home or apartment for a moment. It contains all your property, and theoretically,
it’s a private space for you and your family. Most likely, the door has a lock of some kind
that restricts entry, and, with the use of a key, only you and other authorized persons are able
to enter.
In this example, the door is very much like the remote access device in the network. It
provides a gateway between the outside world and the home—in this case, the corporate
network. The electronic door also has a key of sorts—frequently a username and password.
Access control
defines the manner in which these metaphorical keys are allocated and used;
also, it defines what each person who enters the system can do.
Cisco access control solutions are used to implement the security policies of the network—
specifically, the remote access connectivity. These solutions are targeted for a wide variety of
platforms and functions. You will find Cisco access solutions for several platforms, including
Windows NT and Unix.
Consider the following components used in remote access:
Clients
In Cisco access control, a client is typically a remote user using a dial-in connection
like the one that would be found on an asynchronous or an ISDN connection. These clients can
use different forms of security and authentication, including CHAP and PAP (discussed in
Chapter 24), or they can use remote client software, such as CiscoRemote. In addition, hardware-
based tokens can be used to increase security—the tokens do this by calculating the proper
response to a one-time challenge from the access server.
RADIUS and token-based authentication usually require the use of PAP, which
passes the password in cleartext and is less secure than CHAP.
Access servers
Clients connect to
access servers
, which provide the far end of a connection as
viewed from the remote user’s perspective. Stated another way, the access server is the front
door to the network for remote users. The Cisco IOS and other software, including Cisco
Broadband Operating System (CBOS), can provide varying degrees of security, including dialer
profiles, access control lists (ACLs), and encryption.
To communicate between security servers and access servers, new protocols
were developed, including TACACS+, RADIUS, and Kerberos.
Security servers
Security servers provide a centralized means of controlling policy and storing
account information. This can greatly simplify administration—similar to the way that Domain
Name Server (DNS) eases name-to-address resolution. Recall that before DNS, each workstation
was populated with a hosts file, which had to be modified for each change. DNS enabled hosts to
query a single server for the resolution. Security servers operate in much the same manner—rather
than storing usernames and passwords on each router, they can be stored on the server and queried
by the network device when needed. Cisco’s security server offering is called CiscoSecure, and
it operates on Unix and Windows NT platforms. CiscoSecure is discussed in the next section.
Protocols for centralized authentication
CHAP and PAP were designed for use on serial connections,
making them unsuitable for Ethernet and other LAN technologies.
it’s a private space for you and your family. Most likely, the door has a lock of some kind
that restricts entry, and, with the use of a key, only you and other authorized persons are able
to enter.
In this example, the door is very much like the remote access device in the network. It
provides a gateway between the outside world and the home—in this case, the corporate
network. The electronic door also has a key of sorts—frequently a username and password.
Access control
defines the manner in which these metaphorical keys are allocated and used;
also, it defines what each person who enters the system can do.
Cisco access control solutions are used to implement the security policies of the network—
specifically, the remote access connectivity. These solutions are targeted for a wide variety of
platforms and functions. You will find Cisco access solutions for several platforms, including
Windows NT and Unix.
Consider the following components used in remote access:
Clients
In Cisco access control, a client is typically a remote user using a dial-in connection
like the one that would be found on an asynchronous or an ISDN connection. These clients can
use different forms of security and authentication, including CHAP and PAP (discussed in
Chapter 24), or they can use remote client software, such as CiscoRemote. In addition, hardware-
based tokens can be used to increase security—the tokens do this by calculating the proper
response to a one-time challenge from the access server.
RADIUS and token-based authentication usually require the use of PAP, which
passes the password in cleartext and is less secure than CHAP.
Access servers
Clients connect to
access servers
, which provide the far end of a connection as
viewed from the remote user’s perspective. Stated another way, the access server is the front
door to the network for remote users. The Cisco IOS and other software, including Cisco
Broadband Operating System (CBOS), can provide varying degrees of security, including dialer
profiles, access control lists (ACLs), and encryption.
To communicate between security servers and access servers, new protocols
were developed, including TACACS+, RADIUS, and Kerberos.
Security servers
Security servers provide a centralized means of controlling policy and storing
account information. This can greatly simplify administration—similar to the way that Domain
Name Server (DNS) eases name-to-address resolution. Recall that before DNS, each workstation
was populated with a hosts file, which had to be modified for each change. DNS enabled hosts to
query a single server for the resolution. Security servers operate in much the same manner—rather
than storing usernames and passwords on each router, they can be stored on the server and queried
by the network device when needed. Cisco’s security server offering is called CiscoSecure, and
it operates on Unix and Windows NT platforms. CiscoSecure is discussed in the next section.
Protocols for centralized authentication
CHAP and PAP were designed for use on serial connections,
making them unsuitable for Ethernet and other LAN technologies.
Network Address Translation and Port Address Translation Exam Essentials
Understand what NAT and PAT are and how to use them. NAT is a technology, specified
in RFC 1631, that is used to hide network addresses behind a single IP address or multiple IP
addresses. A company can use IP addresses set aside by RFC 1918 on their internal networks
and use single or multiple Internet-routable IP addresses to connect their company to the network.
PAT is like using NAT through a single IP address.
Know the advantages of NAT and PAT. The advantages of NAT and PAT are that they
enable an entire network to hide behind IP addresses. They provide a certain level of security
and enable a company to change ISPs quickly and painlessly. They also provide a primitive loadbalancing
mechanism between multiple hosts performing the same function.
Know the disadvantages of NAT and PAT. One disadvantage of using NAT and PAT is that
some protocols will not work because they carry IP address information in the payload of the
packet. In addition, NAT and PAT do not provide end-to-end significance for the IP address.
Cisco IOS will correct some of these problems with the most popular protocols, but it cannot
cover them all. Finally, a significant delay occurs in translating IP addresses, which introduces
latency in the communication path.
Understand how to configure NAT and PAT on a Cisco router. One option when configuring
NAT is to use dynamic NAT using a pool of IP addresses or through an interface. You can
also reuse those IP addresses with the overload parameter. PAT uses only a single IP address.
Another option is to configure a static translation from an outside IP address to an internal IP
address. PAT can also be configured with static translations, but they are based on TCP and
UDP port numbers and not on IP addresses only.
Know the troubleshooting techniques for NAT and PAT. The commands used to troubleshoot
NAT are show ip nat translation with the optional verbose parameter, and debug
ip nat, which logs NAT events as they occur on the router. For PAT, the only command used
to show troubleshooting information is the show ip pat command on the 700 series router.
in RFC 1631, that is used to hide network addresses behind a single IP address or multiple IP
addresses. A company can use IP addresses set aside by RFC 1918 on their internal networks
and use single or multiple Internet-routable IP addresses to connect their company to the network.
PAT is like using NAT through a single IP address.
Know the advantages of NAT and PAT. The advantages of NAT and PAT are that they
enable an entire network to hide behind IP addresses. They provide a certain level of security
and enable a company to change ISPs quickly and painlessly. They also provide a primitive loadbalancing
mechanism between multiple hosts performing the same function.
Know the disadvantages of NAT and PAT. One disadvantage of using NAT and PAT is that
some protocols will not work because they carry IP address information in the payload of the
packet. In addition, NAT and PAT do not provide end-to-end significance for the IP address.
Cisco IOS will correct some of these problems with the most popular protocols, but it cannot
cover them all. Finally, a significant delay occurs in translating IP addresses, which introduces
latency in the communication path.
Understand how to configure NAT and PAT on a Cisco router. One option when configuring
NAT is to use dynamic NAT using a pool of IP addresses or through an interface. You can
also reuse those IP addresses with the overload parameter. PAT uses only a single IP address.
Another option is to configure a static translation from an outside IP address to an internal IP
address. PAT can also be configured with static translations, but they are based on TCP and
UDP port numbers and not on IP addresses only.
Know the troubleshooting techniques for NAT and PAT. The commands used to troubleshoot
NAT are show ip nat translation with the optional verbose parameter, and debug
ip nat, which logs NAT events as they occur on the router. For PAT, the only command used
to show troubleshooting information is the show ip pat command on the 700 series router.
Monitoring PAT
Monitoring PAT
To monitor PAT and view the configuration settings, use the show ip pat command. When
monitoring PAT, you can view the number of packets dropped, the timeouts, and the service or
IP address using each individual TCP port. When you configure a Cisco 765 with the configuration
shown in the real-world scenario, you should see output similar to the following example
using the show ip pat command:
765:user1>show ip pat
Dropped - icmp 0, udp 0, tcp 0, map 0, frag 0
Timeout - udp 5 minutes, tcp 30 minutes
Port handlers [no default]:
Port Handler Service
-------------------------------------
21 Router FTP
23 Router TELNET
67 Router DHCP Server
68 Router DHCP Client
69 Router TFTP
80 10.1.2.21 HTTP
161 Router SNMP
162 Router SNMP-TRAP
520 Router RIP
To monitor PAT and view the configuration settings, use the show ip pat command. When
monitoring PAT, you can view the number of packets dropped, the timeouts, and the service or
IP address using each individual TCP port. When you configure a Cisco 765 with the configuration
shown in the real-world scenario, you should see output similar to the following example
using the show ip pat command:
765:user1>show ip pat
Dropped - icmp 0, udp 0, tcp 0, map 0, frag 0
Timeout - udp 5 minutes, tcp 30 minutes
Port handlers [no default]:
Port Handler Service
-------------------------------------
21 Router FTP
23 Router TELNET
67 Router DHCP Server
68 Router DHCP Client
69 Router TFTP
80 10.1.2.21 HTTP
161 Router SNMP
162 Router SNMP-TRAP
520 Router RIP
700 Series Router for Internet Access
Configuring a Typical Office with a 700 Series Router for Internet Access
Company XYZ has decided it’s time to get into this Internet thingy. The company has determined
that their employees need to have Internet access to do research and send e-mail to their
suppliers and customers. There is an internal web server that outside clients and employees
will need to access from the Internet. They have contracted with you to set up a low-cost solution
that will enable them to have access to the Internet from an ISDN line.
You have come up with the following configuration, which enables IP unnumbered across the
WAN connection, enables DHCP server functionality, creates a port handler for the internal web
server at IP address 10.1.2.21, and enables PAT on a Cisco 765 router. It will set the IP address
on the router to 10.1.2.1 and then create a username—XYZ—that was supplied by the ISP. The
following lists the entire configuration on a 765 router. The underlined commands are those
discussed in the previous section:
>set systemname XYZ
XYZ> set switch ni-1
XYZ> set 1 spid 80155511110101
XYZ> set 2 spid 80155522220101
XYZ> set 1 directory 5551111
XYZ> set 2 directory 5552222
XYZ> set dhcp server
XYZ> set dhcp address 10.1.2.2 100
XYZ> set dhcp netmask 255.255.255.0
XYZ> set dhcp gateway primary 10.1.2.1
XYZ> set dhcp dns primary 200.1.1.48
XYZ> set dhcp wins primary 200.1.1.49
XYZ> set dhcp domain mydomain
XYZ> cd lan
XYZ:LAN> set bridging off
XYZ:LAN> set ip routing on
XYZ:LAN> set ip address 10.1.2.1
XYZ:LAN> set ip netmask 255.255.255.0
XYZ:LAN> cd
XYZ> set user ISP
XYZ:ISP> set ppp clientname XYZ
XYZ:ISP> set ppp secret client
Enter new Password: sybex1
Re-Type new Password: sybex1
XYZ:ISP> set ppp password client
Enter new Password: sybex1
Re-Type new Password: sybex1
XYZ:ISP> set bridging off
XYZ:ISP> set ip routing on
XYZ:ISP> set ip rip update off
XYZ:ISP> set ip route destination 0.0.0.0/0 gateway 0.0.0.0
XYZ:ISP> set 1 number 18015553333
XYZ:ISP> set 2 number 18015553333
XYZ:ISP> set ip pat on
XYZ:ISP> set ip pat porthandler http 10.1.2.21
XYZ:ISP> set ppp address negotiation local on
XYZ:ISP> set ppp authentication outgoing none
XYZ:ISP> set timeout 300
XYZ:ISP> set active
With this configuration, a router in XYZ’s office will be able to dial up to the ISP and place their
office on the Internet by using PAT through a single IP address. This will also allow outside
clients and employees access to their web server by using the HTTP port handler.
Company XYZ has decided it’s time to get into this Internet thingy. The company has determined
that their employees need to have Internet access to do research and send e-mail to their
suppliers and customers. There is an internal web server that outside clients and employees
will need to access from the Internet. They have contracted with you to set up a low-cost solution
that will enable them to have access to the Internet from an ISDN line.
You have come up with the following configuration, which enables IP unnumbered across the
WAN connection, enables DHCP server functionality, creates a port handler for the internal web
server at IP address 10.1.2.21, and enables PAT on a Cisco 765 router. It will set the IP address
on the router to 10.1.2.1 and then create a username—XYZ—that was supplied by the ISP. The
following lists the entire configuration on a 765 router. The underlined commands are those
discussed in the previous section:
>set systemname XYZ
XYZ> set switch ni-1
XYZ> set 1 spid 80155511110101
XYZ> set 2 spid 80155522220101
XYZ> set 1 directory 5551111
XYZ> set 2 directory 5552222
XYZ> set dhcp server
XYZ> set dhcp address 10.1.2.2 100
XYZ> set dhcp netmask 255.255.255.0
XYZ> set dhcp gateway primary 10.1.2.1
XYZ> set dhcp dns primary 200.1.1.48
XYZ> set dhcp wins primary 200.1.1.49
XYZ> set dhcp domain mydomain
XYZ> cd lan
XYZ:LAN> set bridging off
XYZ:LAN> set ip routing on
XYZ:LAN> set ip address 10.1.2.1
XYZ:LAN> set ip netmask 255.255.255.0
XYZ:LAN> cd
XYZ> set user ISP
XYZ:ISP> set ppp clientname XYZ
XYZ:ISP> set ppp secret client
Enter new Password: sybex1
Re-Type new Password: sybex1
XYZ:ISP> set ppp password client
Enter new Password: sybex1
Re-Type new Password: sybex1
XYZ:ISP> set bridging off
XYZ:ISP> set ip routing on
XYZ:ISP> set ip rip update off
XYZ:ISP> set ip route destination 0.0.0.0/0 gateway 0.0.0.0
XYZ:ISP> set 1 number 18015553333
XYZ:ISP> set 2 number 18015553333
XYZ:ISP> set ip pat on
XYZ:ISP> set ip pat porthandler http 10.1.2.21
XYZ:ISP> set ppp address negotiation local on
XYZ:ISP> set ppp authentication outgoing none
XYZ:ISP> set timeout 300
XYZ:ISP> set active
With this configuration, a router in XYZ’s office will be able to dial up to the ISP and place their
office on the Internet by using PAT through a single IP address. This will also allow outside
clients and employees access to their web server by using the HTTP port handler.
Configuring PAT
Configuring PAT
The PAT feature enables local hosts with designated private IP addresses to communicate with
the outside world. Basically, the router translates the source address of the IP header into a global,
unique IP address before the packet is forwarded to the outside network. Likewise, IP packets
returning will go through address translations again to the designated private IP addresses
where the communication originated.
When PAT is enabled, RIP packet transmission is automatically disabled to prevent leaking
private IP addresses to the outside network.
To enable PAT, the two commands that you need are as follows:
set ip pat on This command enables PAT and must be configured before the set ip pat
porthandler command can be used.
set ip pat porthandler The port handler translates a public TCP or UDP port to a private IP
address and port. When a packet is received from the outside, the router compares the port number
with an internally configured port handler list of up to 15 entries. If a port handler is defined
for this port, it routes the packet to the appropriate port handler (internal IP address). If a default
port handler is defined, it routes the packet there. The possible parameters are as follows:
default enables the port handler for all well-known ports, except ports specifically
assigned a handler.
telnet enables the port handler for the Telnet protocol on port 23.
ftp enables the port handler for File Transport Protocol (FTP) and uses TCP protocol
port 21.
smtp enables the port handler for Simple Mail Transfer Protocol (SMTP) and uses TCP
protocol port 25.
wins enables the port handler for NetBIOS session service on port 139.
http enables the port handler for World Wide Web–HTTP service and secure-HTTP
port 80 or 443.
off disables a certain port handler.
port configures a custom port handler for a port not normally considered a well-known
port. Remember that only 15 port handlers can be configured at once.
All parameters are followed by the appropriate IP address.
The PAT feature enables local hosts with designated private IP addresses to communicate with
the outside world. Basically, the router translates the source address of the IP header into a global,
unique IP address before the packet is forwarded to the outside network. Likewise, IP packets
returning will go through address translations again to the designated private IP addresses
where the communication originated.
When PAT is enabled, RIP packet transmission is automatically disabled to prevent leaking
private IP addresses to the outside network.
To enable PAT, the two commands that you need are as follows:
set ip pat on This command enables PAT and must be configured before the set ip pat
porthandler command can be used.
set ip pat porthandler The port handler translates a public TCP or UDP port to a private IP
address and port. When a packet is received from the outside, the router compares the port number
with an internally configured port handler list of up to 15 entries. If a port handler is defined
for this port, it routes the packet to the appropriate port handler (internal IP address). If a default
port handler is defined, it routes the packet there. The possible parameters are as follows:
default enables the port handler for all well-known ports, except ports specifically
assigned a handler.
telnet enables the port handler for the Telnet protocol on port 23.
ftp enables the port handler for File Transport Protocol (FTP) and uses TCP protocol
port 21.
smtp enables the port handler for Simple Mail Transfer Protocol (SMTP) and uses TCP
protocol port 25.
wins enables the port handler for NetBIOS session service on port 139.
http enables the port handler for World Wide Web–HTTP service and secure-HTTP
port 80 or 443.
off disables a certain port handler.
port configures a custom port handler for a port not normally considered a well-known
port. Remember that only 15 port handlers can be configured at once.
All parameters are followed by the appropriate IP address.
Disadvantages of PAT
Disadvantages of PAT
Using PAT has some disadvantages because it takes away end-to-end reachability. These disadvantages
are as follows:
You cannot use Ping from an outside host to a host in the private network.
Telnet from an outside host to an inside host is not forwarded unless the Telnet port handler
is configured.
Only one FTP server and one Telnet server are supported on the inside network.
Packets destined for the router itself and not an inside network IP address, such as DHCP,
SNMP, PING, or TFTP, are not rejected or filtered by PAT.
Because the 700 series is a low-end solution, if more than 12 PCs try to boot up simultaneously
on the inside, one or more might get an error message about not being able to
access the server.
The PAT table is limited to 400 entries for the inside machines to share. If TCP translations
are set up and the TCP timeouts are kept alive, no more than 400 machines can get to the
outside world at any one time.
The Cisco 700 series router with PAT enabled does not handle any fragmented FTP packets;
this needs to be noted when troubleshooting.
Some well-known ports cannot have port handlers defined. They include the following:
DHCP client ports used by the router for getting DHCP server responses
WINS NetBIOS ports used by the inside network clients operating Windows 95 PCs to
get WINS information
Using PAT has some disadvantages because it takes away end-to-end reachability. These disadvantages
are as follows:
You cannot use Ping from an outside host to a host in the private network.
Telnet from an outside host to an inside host is not forwarded unless the Telnet port handler
is configured.
Only one FTP server and one Telnet server are supported on the inside network.
Packets destined for the router itself and not an inside network IP address, such as DHCP,
SNMP, PING, or TFTP, are not rejected or filtered by PAT.
Because the 700 series is a low-end solution, if more than 12 PCs try to boot up simultaneously
on the inside, one or more might get an error message about not being able to
access the server.
The PAT table is limited to 400 entries for the inside machines to share. If TCP translations
are set up and the TCP timeouts are kept alive, no more than 400 machines can get to the
outside world at any one time.
The Cisco 700 series router with PAT enabled does not handle any fragmented FTP packets;
this needs to be noted when troubleshooting.
Some well-known ports cannot have port handlers defined. They include the following:
DHCP client ports used by the router for getting DHCP server responses
WINS NetBIOS ports used by the inside network clients operating Windows 95 PCs to
get WINS information
Using Port Address Translation (PAT)
Using Port Address Translation (PAT)
If you wish to enable address translation on the 700 series router, you use Port Address Translation
(PAT). PAT is a subset of NAT and is the only address translation feature on the Cisco
700 series of routers. PAT uses TCP ports to enable an entire network to use only one globally
routable IP address in the network. PAT is similar to overloading with traditional NAT.
The Cisco 700 series routers with release 4 software and higher support PAT, which enables
local hosts on an inside IP network to communicate to an outside IP network such as the Internet.
Traffic destined for an outside IP address on the other side of a border router will have its
source IP address translated before the packet is forwarded to the outside network. IP packets
returning to the inside network will have their destination IP addresses translated back to the
original source IP addresses on the inside network.
PAT conserves network addresses by enabling a single Internet-routable IP address to be
assigned to an entire LAN. All WAN traffic is usually mapped to a single IP address, which is
the ISDN-side IP address of the Cisco 700 series router. Because all the traffic on the outside network
appears to come from the Cisco 700, the inside network appears invisible to the outside
network or Internet.
You should configure a static IP address and port if remote users need to access a specific
server on the inside network. PAT will allow packets with a specific well-known port number
to get through, such as FTP or Telnet. This feature is known as a default port handler.
PAT is also sometimes referred to as NAT overload.
If you wish to enable address translation on the 700 series router, you use Port Address Translation
(PAT). PAT is a subset of NAT and is the only address translation feature on the Cisco
700 series of routers. PAT uses TCP ports to enable an entire network to use only one globally
routable IP address in the network. PAT is similar to overloading with traditional NAT.
The Cisco 700 series routers with release 4 software and higher support PAT, which enables
local hosts on an inside IP network to communicate to an outside IP network such as the Internet.
Traffic destined for an outside IP address on the other side of a border router will have its
source IP address translated before the packet is forwarded to the outside network. IP packets
returning to the inside network will have their destination IP addresses translated back to the
original source IP addresses on the inside network.
PAT conserves network addresses by enabling a single Internet-routable IP address to be
assigned to an entire LAN. All WAN traffic is usually mapped to a single IP address, which is
the ISDN-side IP address of the Cisco 700 series router. Because all the traffic on the outside network
appears to come from the Cisco 700, the inside network appears invisible to the outside
network or Internet.
You should configure a static IP address and port if remote users need to access a specific
server on the inside network. PAT will allow packets with a specific well-known port number
to get through, such as FTP or Telnet. This feature is known as a default port handler.
PAT is also sometimes referred to as NAT overload.
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