We’ve talked formally about CIR—we even presented a calculation—but what does it really mean?
As we have said, the acronym itself stands for committed information rate, which really doesn’t
seem that difficult to understand. But there seems to be widespread misinterpretation of this concept,
especially by some service providers, so let’s attempt to figure the whole thing out.
We’ve discussed terms such as burst rate and phrases such as bursting above your CIR, but
these terms can be misleading. They were devised by network engineers who assumed—you
know what that leads to—that we wouldn’t understand hardcore network-engineering concepts,
so they tried to put them in layman’s terms and botched the whole thing up. In reality,
you’re always “bursting” to your line speed because Frame Relay is an HDLC protocol and
there’s no other way to make it work.
HDLC is a synchronous protocol (which means that the data is synchronized to a clock) that
sends data with a standardized framing and checksum technique. When a frame is transmitted,
the data must be contiguous; that is, there cannot be any holes or spaces between bytes of
data. So if you’re transmitting 500 bytes of data, you can’t send 250 and then wait for a while
and then send the rest. It has to go out as one big chunk. The Frame Relay expression bursting
over your CIR comes into play because there is no way to slow down the data or to change the
length of the chunk after you start transmitting; you just send until you are finished. If you happen
to send too much data because your data chunk is larger than the allotment, you’ve
bursted over your CIR.
So what is the big deal about CIR, then? And why, when you buy Frame Relay from a company
like Qwest, do they quote you a CIR? CIR is the “worst-case” throughput that the Frame Relay
network provider attempts to guarantee. It’s like a restaurant guaranteeing that you’ll always
be able to eat a certain amount of food from its buffet. Like the restaurant, the Frame Relay network
provider can’t guarantee that you’ll always be able to transmit at the CIR (take the case
when everyone on the network happens to be transmitting at once), but they can guarantee it
over a reasonable time span (usually over a span of seconds). Basically, the network backbone
is engineered to handle reasonable loads—just like the number of lanes in a highway. Given a
certain amount of traffic, the data should flow through the backbone without delay. At times,
when unusually heavy traffic exists, you have what is called congestion.
IT Certification CCIE,CCNP,CCIP,CCNA,CCSP,Cisco Network Optimization and Security Tips
Frame Relay Summary
Frame Relay is one of the most popular WAN protocols in the world. This technology
will become even more critical as corporations stretch their networks globally and the Internet
becomes more pervasive.
To become a successful CCNP, you need to understand the Frame Relay protocol. This technology
makes up the majority of the world’s non-dedicated circuits, and its importance cannot
be underestimated.
Frame Relay is the distant cousin to X.25, without some of X.25’s overhead. It does provide
congestion notification, which can be used with traffic shaping to help traffic response. Like
X.25, Frame Relay provides for permanent and switched virtual circuits.
LMI is an extension to the Frame Relay protocol developed by the Frame Relay Forum and
is used to provide management for virtual circuits. This makes the management of DLCI information
easier for the network administrator.
Cisco provides for a mechanism to enable a multipoint interface such as Frame Relay to look
like multiple virtual point-to-point or multipoint interfaces called subinterfaces. Point-to-point
subinterfaces can be used to solve problems caused by distance-vector routing protocols running
over multipoint interfaces.
Setting up a Cisco router as a Frame Relay switch is not something that you would do often,
but it is a useful feature when you are working in a lab environment. There are many troubleshooting
commands that can be used to verify the configuration of Frame Relay on a Cisco
router. They can be used to see the DLCI-to-Network-layer-address mapping and the current
state of LMI on the router. Frame Relay is a technology used in many networks, and mastering
its configuration and operation will take you far in your networking career.
will become even more critical as corporations stretch their networks globally and the Internet
becomes more pervasive.
To become a successful CCNP, you need to understand the Frame Relay protocol. This technology
makes up the majority of the world’s non-dedicated circuits, and its importance cannot
be underestimated.
Frame Relay is the distant cousin to X.25, without some of X.25’s overhead. It does provide
congestion notification, which can be used with traffic shaping to help traffic response. Like
X.25, Frame Relay provides for permanent and switched virtual circuits.
LMI is an extension to the Frame Relay protocol developed by the Frame Relay Forum and
is used to provide management for virtual circuits. This makes the management of DLCI information
easier for the network administrator.
Cisco provides for a mechanism to enable a multipoint interface such as Frame Relay to look
like multiple virtual point-to-point or multipoint interfaces called subinterfaces. Point-to-point
subinterfaces can be used to solve problems caused by distance-vector routing protocols running
over multipoint interfaces.
Setting up a Cisco router as a Frame Relay switch is not something that you would do often,
but it is a useful feature when you are working in a lab environment. There are many troubleshooting
commands that can be used to verify the configuration of Frame Relay on a Cisco
router. They can be used to see the DLCI-to-Network-layer-address mapping and the current
state of LMI on the router. Frame Relay is a technology used in many networks, and mastering
its configuration and operation will take you far in your networking career.
Configuring Traffic Shaping
To configure Frame Relay traffic shaping, you must first enter the map class configuration mode
so you can define a map class. You enter the map class with the global configuration command
map-class frame-relay name. The name parameter is the name you use to apply the map class
to the VC where you want traffic shaping performed. The command looks like the following:
RouterA#config t
RouterA(config)#map-class frame-relay scott
RouterA(config-map-class)#
Notice that the map-class frame-relay scott command changes the prompt to
config-map-class. This enables you to configure the parameters for your map class.
The map class is used to define the average and peak rates allowed in each VC associated
with the map class. The map class mechanism enables you to specify that the router can dynamically
fluctuate the rate at which it sends traffic, depending on the BECNs received. It also
enables you to configure queuing on a per-VC basis.
To define the average and peak rate for links that are faster than the receiving link can handle,
use the following command:
RouterA(config-map-class)#frame-relay traffic-rate average [peak]
The average parameter sets the average rate in bits per second, which is your CIR. Now, how
do you calculate the peak value? First, start with the EIR. The EIR is the average rate over which
bits will be marked with DE and is given by the formula EIR = Be/Tc, with Be being excessive
burst and Tc representing the committed rate measurement interval. The peak value is then calculated
by taking the CIR plus EIR, or peak = CIR + EIR.
The peak parameter is optional. An example of a line is as follows:
RouterA(config-map-class)#frame-relay traffic-rate 9600 18000
To specify that the router should dynamically fluctuate the rate at which it is sending traffic
depending on the number of BECNs received, use the following command:
RouterA(config-map-class)#frame-relay adaptive-shaping becn
To set bandwidth usage for protocols, you can configure traffic shaping to use queuing on
a per-VC basis. To perform this function, use the following commands:
RouterA(config-map-class)#frame-relay custom-queue-list number
RouterA(config-map-class)#frame-relay priority-group number
You can use either command, depending on the type of queuing you are using. The number
parameter at the end of the command is the queue list number. A detailed discussion of queuing
is presented in the next chapter.
After the map class parameters are completed, you then need to configure the traffic shaping
on the interface you want. The following commands are used to perform traffic shaping on an
interface and to apply the map class and its parameters to a subinterface and, by association, its
corresponding VC:
RouterA#config t
RouterA(config)#interface serial0
RouterA(config-if)#frame-relay traffic-shaping
RouterA(config-if)#interface serial0.16 point-to-point
RouterA(config-subif#frame-relay class scott
RouterA(config-subif)#frame-relay interface-dlci 16
You first must enable traffic shaping and per-VC queuing on the interface with the
frame-relay traffic-shaping command. You can then go to the interface or subinterface
and assign the map class by using the frame-relay class name command. The example
just shown uses the name scott because that is the name of the map class defined in the
earlier example.
After you have completed the configuration, use the show running-config and the show
frame-relay pvc commands to verify the configuration.
so you can define a map class. You enter the map class with the global configuration command
map-class frame-relay name. The name parameter is the name you use to apply the map class
to the VC where you want traffic shaping performed. The command looks like the following:
RouterA#config t
RouterA(config)#map-class frame-relay scott
RouterA(config-map-class)#
Notice that the map-class frame-relay scott command changes the prompt to
config-map-class. This enables you to configure the parameters for your map class.
The map class is used to define the average and peak rates allowed in each VC associated
with the map class. The map class mechanism enables you to specify that the router can dynamically
fluctuate the rate at which it sends traffic, depending on the BECNs received. It also
enables you to configure queuing on a per-VC basis.
To define the average and peak rate for links that are faster than the receiving link can handle,
use the following command:
RouterA(config-map-class)#frame-relay traffic-rate average [peak]
The average parameter sets the average rate in bits per second, which is your CIR. Now, how
do you calculate the peak value? First, start with the EIR. The EIR is the average rate over which
bits will be marked with DE and is given by the formula EIR = Be/Tc, with Be being excessive
burst and Tc representing the committed rate measurement interval. The peak value is then calculated
by taking the CIR plus EIR, or peak = CIR + EIR.
The peak parameter is optional. An example of a line is as follows:
RouterA(config-map-class)#frame-relay traffic-rate 9600 18000
To specify that the router should dynamically fluctuate the rate at which it is sending traffic
depending on the number of BECNs received, use the following command:
RouterA(config-map-class)#frame-relay adaptive-shaping becn
To set bandwidth usage for protocols, you can configure traffic shaping to use queuing on
a per-VC basis. To perform this function, use the following commands:
RouterA(config-map-class)#frame-relay custom-queue-list number
RouterA(config-map-class)#frame-relay priority-group number
You can use either command, depending on the type of queuing you are using. The number
parameter at the end of the command is the queue list number. A detailed discussion of queuing
is presented in the next chapter.
After the map class parameters are completed, you then need to configure the traffic shaping
on the interface you want. The following commands are used to perform traffic shaping on an
interface and to apply the map class and its parameters to a subinterface and, by association, its
corresponding VC:
RouterA#config t
RouterA(config)#interface serial0
RouterA(config-if)#frame-relay traffic-shaping
RouterA(config-if)#interface serial0.16 point-to-point
RouterA(config-subif#frame-relay class scott
RouterA(config-subif)#frame-relay interface-dlci 16
You first must enable traffic shaping and per-VC queuing on the interface with the
frame-relay traffic-shaping command. You can then go to the interface or subinterface
and assign the map class by using the frame-relay class name command. The example
just shown uses the name scott because that is the name of the map class defined in the
earlier example.
After you have completed the configuration, use the show running-config and the show
frame-relay pvc commands to verify the configuration.
Using Traffic-Shaping Techniques
The following list outlines the traffic-shaping techniques used with Frame Relay:
To control the access rate transmitted on a Cisco router, you can configure a peak rate to
limit outbound traffic to either the CIR or excess information rate (EIR).
You can configure BECN support on a per-VC basis, which will enable the router to then
monitor BECNs and throttle traffic based on BECN-designated packets.
Queuing can be used for support at the VC level. Priority, custom, and weighted fair queuing
(WFQ) can be used. This gives you more control over traffic flow on individual VCs.
It’s also important to understand when you would use traffic shaping with Frame Relay. The
following list explains this:
Use traffic shaping when one site, such as the corporate office, has a higher speed line (for
example, a T1), and the remote branches have slower lines (for example, 56Kbps). This
connection would cause bottlenecks on each VC and would result in poor response times
for time-sensitive traffic such as SNA and Telnet. This can cause packets to be dropped. By
using traffic shaping at the corporate office, you can improve response on each VC.
Traffic shaping is also helpful on a router with many subinterfaces. Because these subinterfaces
will use traffic as fast as the physical link allows, you can use rate enforcement on the subinterface
to match the CIR of the VC. This means you can preallocate bandwidth to each VC.
Traffic shaping can be used to throttle back transmission on a Frame Relay network that
is constantly congested. This can help prevent packet loss and is done on a per-VC basis.
Traffic shaping is used effectively if you have multiple Network layer protocols and want
to queue each protocol to allocate bandwidth effectively. Since IOS version 11.2, queuing
can be performed at the VC level.
To control the access rate transmitted on a Cisco router, you can configure a peak rate to
limit outbound traffic to either the CIR or excess information rate (EIR).
You can configure BECN support on a per-VC basis, which will enable the router to then
monitor BECNs and throttle traffic based on BECN-designated packets.
Queuing can be used for support at the VC level. Priority, custom, and weighted fair queuing
(WFQ) can be used. This gives you more control over traffic flow on individual VCs.
It’s also important to understand when you would use traffic shaping with Frame Relay. The
following list explains this:
Use traffic shaping when one site, such as the corporate office, has a higher speed line (for
example, a T1), and the remote branches have slower lines (for example, 56Kbps). This
connection would cause bottlenecks on each VC and would result in poor response times
for time-sensitive traffic such as SNA and Telnet. This can cause packets to be dropped. By
using traffic shaping at the corporate office, you can improve response on each VC.
Traffic shaping is also helpful on a router with many subinterfaces. Because these subinterfaces
will use traffic as fast as the physical link allows, you can use rate enforcement on the subinterface
to match the CIR of the VC. This means you can preallocate bandwidth to each VC.
Traffic shaping can be used to throttle back transmission on a Frame Relay network that
is constantly congested. This can help prevent packet loss and is done on a per-VC basis.
Traffic shaping is used effectively if you have multiple Network layer protocols and want
to queue each protocol to allocate bandwidth effectively. Since IOS version 11.2, queuing
can be performed at the VC level.
Frame Relay Traffic Shaping
Traffic shaping on Frame Relay provides different capabilities, and because this information
might be covered on the exam, it is important that you can describe each one. On production
networks, this information can help you understand whether switch problems are occurring.
In this section, you will learn about traffic-shaping techniques and when to use them. You’ll
then learn how to configure traffic shaping.
might be covered on the exam, it is important that you can describe each one. On production
networks, this information can help you understand whether switch problems are occurring.
In this section, you will learn about traffic-shaping techniques and when to use them. You’ll
then learn how to configure traffic shaping.
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