As mentioned, each layer of the OSI model utilizes specific protocols that enable the layer to perform
the necessary functions and communicate with adjacent layers. Each protocol has specific
properties based on the functions that it needs to accomplish. Throughout all seven layers, there
are two major protocol classifications: connection-oriented and connectionless. 1098.2.
IT Certification CCIE,CCNP,CCIP,CCNA,CCSP,Cisco Network Optimization and Security Tips
The OSI Reference Model
This section is a review of the OSI model, which was originally discussed in
CCNA: Cisco Certified
Network Associate Study Guide, 4th ed
., by Todd Lammle (Sybex, 2004). The
OSI model
(the Open Systems Interconnection reference model) is the template used to design applications
or protocols that allow nonhomogenous computers or networks to communicate with one
another. The ISO (International Organization for Standardization) developed the OSI model.
The OSI model consists of seven layers. Each layer communicates directly with its adjacent layers,
as well as with the corresponding layer of the destination system (depicted in Figure 36.1).
Communication between layers facilitates the transfer of data up and down the OSI model. Communication
between the corresponding layers of the source system and the destination system
enables two heterogeneous networks or computers to understand each other.
The OSI template defines the services and roles that each layer is to provide. Because each
layer provides different services and functions, the layers need to communicate so that the data
can be transmitted up and down the seven layers and onto the destination system. The following
list summarizes the responsibility of each of the seven layers, starting from the Physical layer
and working up to the Application layer:
Physical
This layer sends and receives bits with values of 1s and 0s. The Physical layer is
in charge of determining how it sends these values. If the physical connection between two
machines is fiber-optic, then the Physical layer has to use light to transmit the 1s and 0s. If
the connection is electrical, then electrical signals are sent to represent the 1s and 0s.
Data Link
This layer takes all the data that is accumulated as packets are handed from one layer
to the next and then packages it into frames. The Data Link layer equates the Network layer address
(IP address) to a data link address, or MAC address, of the next hop. Once the physical address is
known, the frame is sent to that address. The receiving interface uses the Data Link layer to extract
the packet from the frame, discards the frame, and then sends the packet up to the Network layer.
Network
This layer defines the topology of the network through the use of logical addressing.
Routing protocols use this information to route packets.
Transport
This layer takes care of end-to-end communications. It is responsible for the
connection to the destination system, as well as for packet segmentation and assembly. The
Transport layer includes both connection-oriented and connectionless protocols (for example,
TCP and UDP).
Session
This layer is responsible for coordinating communication among applications, which
it does through dialog-control methods.
Presentation
This layer negotiates syntax, so it is responsible for the proper method of presenting
the data to the Application layer. Some of the Presentation layer functions are compression/
decompression and encryption/decryption of data.
Application
This is the user and application interface. The Application layer is responsible for
data exchange and job management. It also handles file, print, message, database, and application
services.
You saw how the logical data flow of the OSI model works, but look at Figure 36.2, in
which you can see the actual data flow. This figure depicts data that is handed from the Application
layer all the way down to the Physical layer. At that point, the data is transmitted
across any variety of physical media to the next hop, or destination system. Once the 1s and
0s arrive at the Physical layer of the destination system, the information is sent to layer 2 (the
Data Link layer). This layer discards the frame, and then the extracted packet is handed up
to the Network layer. The network packet header is stripped off, and the resulting packet is
handed up to the Transport layer. This process is repeated for each layer until it arrives at the
Application layer.
Now that each layer of the OSI reference model has been explained briefly, you need to
focus on the functions of each layer in detail. This detail provides the necessary background
and information to effectively troubleshoot network problems that occur within specific layers
of the OSI model.
CCNA: Cisco Certified
Network Associate Study Guide, 4th ed
., by Todd Lammle (Sybex, 2004). The
OSI model
(the Open Systems Interconnection reference model) is the template used to design applications
or protocols that allow nonhomogenous computers or networks to communicate with one
another. The ISO (International Organization for Standardization) developed the OSI model.
The OSI model consists of seven layers. Each layer communicates directly with its adjacent layers,
as well as with the corresponding layer of the destination system (depicted in Figure 36.1).
Communication between layers facilitates the transfer of data up and down the OSI model. Communication
between the corresponding layers of the source system and the destination system
enables two heterogeneous networks or computers to understand each other.
The OSI template defines the services and roles that each layer is to provide. Because each
layer provides different services and functions, the layers need to communicate so that the data
can be transmitted up and down the seven layers and onto the destination system. The following
list summarizes the responsibility of each of the seven layers, starting from the Physical layer
and working up to the Application layer:
Physical
This layer sends and receives bits with values of 1s and 0s. The Physical layer is
in charge of determining how it sends these values. If the physical connection between two
machines is fiber-optic, then the Physical layer has to use light to transmit the 1s and 0s. If
the connection is electrical, then electrical signals are sent to represent the 1s and 0s.
Data Link
This layer takes all the data that is accumulated as packets are handed from one layer
to the next and then packages it into frames. The Data Link layer equates the Network layer address
(IP address) to a data link address, or MAC address, of the next hop. Once the physical address is
known, the frame is sent to that address. The receiving interface uses the Data Link layer to extract
the packet from the frame, discards the frame, and then sends the packet up to the Network layer.
Network
This layer defines the topology of the network through the use of logical addressing.
Routing protocols use this information to route packets.
Transport
This layer takes care of end-to-end communications. It is responsible for the
connection to the destination system, as well as for packet segmentation and assembly. The
Transport layer includes both connection-oriented and connectionless protocols (for example,
TCP and UDP).
Session
This layer is responsible for coordinating communication among applications, which
it does through dialog-control methods.
Presentation
This layer negotiates syntax, so it is responsible for the proper method of presenting
the data to the Application layer. Some of the Presentation layer functions are compression/
decompression and encryption/decryption of data.
Application
This is the user and application interface. The Application layer is responsible for
data exchange and job management. It also handles file, print, message, database, and application
services.
You saw how the logical data flow of the OSI model works, but look at Figure 36.2, in
which you can see the actual data flow. This figure depicts data that is handed from the Application
layer all the way down to the Physical layer. At that point, the data is transmitted
across any variety of physical media to the next hop, or destination system. Once the 1s and
0s arrive at the Physical layer of the destination system, the information is sent to layer 2 (the
Data Link layer). This layer discards the frame, and then the extracted packet is handed up
to the Network layer. The network packet header is stripped off, and the resulting packet is
handed up to the Transport layer. This process is repeated for each layer until it arrives at the
Application layer.
Now that each layer of the OSI reference model has been explained briefly, you need to
focus on the functions of each layer in detail. This detail provides the necessary background
and information to effectively troubleshoot network problems that occur within specific layers
of the OSI model.
Protocol Attributes
THE CCNP EXAM TOPICS COVERED IN THIS
BLOG INCLUDE THE FOLLOWING:
Verify network connectivity.
Use the optimal troubleshooting approach in resolving
network problems.
Minimize downtime during troubleshooting.
Use Cisco IOS commands to identify problems.
Determine the layer or layers on which a problem is occurring.
As you know, to successfully troubleshoot network problems, it is
important to have a good understanding of how network components,
including PCs and servers, communicate with each other.
Without this basic knowledge, troubleshooting a network problem is like trying to read a book in
a foreign language. The information is there, but it just isn’t comprehensible. Although the troubleshooting
model discussed in Chapter 33, “Troubleshooting Methodology,” provides the method of
retrieving all the necessary information, the data is useless without an understanding of the information
presented.
This chapter is a review of the protocols used by layers 2, 3, and 4 of the OSI model. We briefly
review the seven layers of the OSI model, and then discuss how they communicate with one another.
We then discuss layer 2 and layer 3 protocols. More specific information on some of the material
covered here can be found in later chapters and is cross-referenced here where appropriate.
BLOG INCLUDE THE FOLLOWING:
Verify network connectivity.
Use the optimal troubleshooting approach in resolving
network problems.
Minimize downtime during troubleshooting.
Use Cisco IOS commands to identify problems.
Determine the layer or layers on which a problem is occurring.
As you know, to successfully troubleshoot network problems, it is
important to have a good understanding of how network components,
including PCs and servers, communicate with each other.
Without this basic knowledge, troubleshooting a network problem is like trying to read a book in
a foreign language. The information is there, but it just isn’t comprehensible. Although the troubleshooting
model discussed in Chapter 33, “Troubleshooting Methodology,” provides the method of
retrieving all the necessary information, the data is useless without an understanding of the information
presented.
This chapter is a review of the protocols used by layers 2, 3, and 4 of the OSI model. We briefly
review the seven layers of the OSI model, and then discuss how they communicate with one another.
We then discuss layer 2 and layer 3 protocols. More specific information on some of the material
covered here can be found in later chapters and is cross-referenced here where appropriate.
End-System Documentation and Troubleshooting Exam Essentials
Know what end-system network configuration tables are and the information they contain.
End-system network configuration tables are used to record key settings of end systems in the
network. Items commonly included in an end-system network configuration table are system
name, system manufacturer/model, CPU speed, RAM, storage, system purpose, media type,
interface speed, VLAN, IP address, default gateway, subnet mask, WINS, DNS, operating system
(including version), network-based applications, high-bandwidth applications, and lowlatency
applications.
Know what end-system network topology diagrams are and the information they contain.
End-system network topology diagrams are graphical representations of the network and are
usually built with many of the same components as the end-system network configuration
tables. Some common components of the end-system network topology diagram are system
name, connection to the network, system purpose, VLAN, IP address, subnet mask, and network
applications.
Know the commands to discover information and troubleshoot end systems. There are Unix
and Windows versions of the discovery and troubleshooting commands, and many of them correlate
directly to Cisco IOS commands. Some of these commands are arp, ifconfig, ipconfig,
netstat, ping, route, telnet, and traceroute.
End-system network configuration tables are used to record key settings of end systems in the
network. Items commonly included in an end-system network configuration table are system
name, system manufacturer/model, CPU speed, RAM, storage, system purpose, media type,
interface speed, VLAN, IP address, default gateway, subnet mask, WINS, DNS, operating system
(including version), network-based applications, high-bandwidth applications, and lowlatency
applications.
Know what end-system network topology diagrams are and the information they contain.
End-system network topology diagrams are graphical representations of the network and are
usually built with many of the same components as the end-system network configuration
tables. Some common components of the end-system network topology diagram are system
name, connection to the network, system purpose, VLAN, IP address, subnet mask, and network
applications.
Know the commands to discover information and troubleshoot end systems. There are Unix
and Windows versions of the discovery and troubleshooting commands, and many of them correlate
directly to Cisco IOS commands. Some of these commands are arp, ifconfig, ipconfig,
netstat, ping, route, telnet, and traceroute.
End-System Documentation and Troubleshooting Summary
End-system documentation is just as important as the network documentation in terms of the overall
documentation strategy. The two main components that make up end-system documentation are
the end-system network configuration table and the end-system network topology table.
End-system network configuration tables are documents that show the key configuration
parameters in place on the end systems in the network. Some of the common items in an endsystem
network configuration table are the system name, system manufacturer/model, CPU
speed, RAM, storage, system purpose, media type, interface speed, VLAN, IP address, default
gateway, subnet mask, WINS, DNS, operating system (including version), network-based applications,
high-bandwidth applications, and low-latency applications. The specific items included
on the end-system network configuration table depend on the purpose of the documentation. In
most cases, the end-system table is kept in a spreadsheet or database format. As is the case with
all the documentation covered in this book, be sure to keep hardcopies of the documents to use
in the event of a network outage.
End-system network topology diagrams are graphical representations of the end systems in
the network. In many cases, they are just additions to the network topology diagram; however,
they can be their own entity. The data included in an end-system network topology diagram is
usually a small subset of that maintained in the end-system network configuration tables. The
topology diagrams are meant to make the network administrator better able to visualize the
path across the network. Some of the standard items that go into an end-system network topology
are system name, connection to the network, system purpose, VLAN, IP address, subnet
mask, and network applications.
Finally, in this chapter we covered a number of commands that can be used to effectively
troubleshoot problems on end systems. These commands include ping and its record route
option, traceroute, arp, route, nbtstat, netstat, and ipconfig. All of these commands
have Windows NT/2000/XP and Unix equivalents, and most have a direct relationship to a
Cisco IOS command.
documentation strategy. The two main components that make up end-system documentation are
the end-system network configuration table and the end-system network topology table.
End-system network configuration tables are documents that show the key configuration
parameters in place on the end systems in the network. Some of the common items in an endsystem
network configuration table are the system name, system manufacturer/model, CPU
speed, RAM, storage, system purpose, media type, interface speed, VLAN, IP address, default
gateway, subnet mask, WINS, DNS, operating system (including version), network-based applications,
high-bandwidth applications, and low-latency applications. The specific items included
on the end-system network configuration table depend on the purpose of the documentation. In
most cases, the end-system table is kept in a spreadsheet or database format. As is the case with
all the documentation covered in this book, be sure to keep hardcopies of the documents to use
in the event of a network outage.
End-system network topology diagrams are graphical representations of the end systems in
the network. In many cases, they are just additions to the network topology diagram; however,
they can be their own entity. The data included in an end-system network topology diagram is
usually a small subset of that maintained in the end-system network configuration tables. The
topology diagrams are meant to make the network administrator better able to visualize the
path across the network. Some of the standard items that go into an end-system network topology
are system name, connection to the network, system purpose, VLAN, IP address, subnet
mask, and network applications.
Finally, in this chapter we covered a number of commands that can be used to effectively
troubleshoot problems on end systems. These commands include ping and its record route
option, traceroute, arp, route, nbtstat, netstat, and ipconfig. All of these commands
have Windows NT/2000/XP and Unix equivalents, and most have a direct relationship to a
Cisco IOS command.
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