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272 Chapter 9: Frame Relay Connection Controlling Traffic Flow
(Be + Bc)
ч 16, or (112,000 + 16,000) ч 16 = 8000, bps per time interval. This forces it back to
CIR (128,000 bps) immediately after the 16 (16(8000)) time intervals with no BECN.
This behavior only occurs when Frame Relay traffic shaping is active. If traffic shaping is not
active, the transmission increment is fixed even though BECNs are not being received.
Using Frame Relay Traffic Shaping
There are a number of instances in which Frame Relay traffic shaping can be useful. For
instance, if your network consists of high-bandwidth links exclusively, traffic shaping can be
beneficial. In mixed bandwidth environments, traffic shaping can also be beneficial. In addition,
any environment in which Voice over Frame Relay is being implemented requires the use of
traffic shaping to adequately configure the network for voice traffic.
In mixed bandwidth environments, traffic shaping can be utilized to protect the lower-speed
links. For example, a T1 link transmitting data to a site in which the bandwidth is only 56 kbps
can be forced into a congestive state quite easily. If traffic shaping is implemented on the faster
side, specifically on the VC directing traffic to that specific site, congestion can be stopped
before it begins.
Frame Relay Traffic Shaping Configuration
Frame Relay traffic shaping is accomplished through the creation of a map class. This map class
can be associated with one or more PVCs. The map class defines the traffic parameters for any
circuits to which it is applied. The command structure for defining the map class is as follows:
RouterA(config)#map-class frame-relay name
The name parameter is an arbitrary value. This is the parameter that is used to tie the map class
to one or more PVCs.
Once the map-class command has been entered, the prompt changes. At this point, it is time to
define the traffic parameters. The average and peak transmission rates can be configured at this
point along with defining whether the router should respond to BECN requests. It is also
possible to define queues to prioritize PVCs. The command structure for defining peak and
average rates is as follows (the peak rate is optional):
RouterA(config-map-class)#frame-relay traffic-rate average [peak]
The average rate is measured in bps (bps) and should be set to the contracted CIR. The peak
rate is also measured in bps. Do not use the frame-relay traffic-rate command if you are
enabling the adaptive-shaping becn command (discussed in the next paragraph). Instead, you
should enable a BECN response.
To specify that the router should respond to BECN requests for throughput reduction, use the
frame-relay adaptive-shaping becn command. Its command structure is as follows:
RouterA(config-map-class)#frame-relay adaptive-shaping becn