8.4 Frame Relay Traffic Shaping
8.4.8 Dynamic enforcement example
Figure illustrates a Frame Relay environment in which different sites might have different speeds for local-loop connections to the Frame Relay cloud. Example shows traffic shaping through a BECN example.

In this environment, the following process can occur:

  1. Central site sends data to router branch site.
  2. One of the switches within the cloud determines that it is getting congested with traffic. In this case, the congested switch sends a BECN packet to the central router.

    Note: Prior to the traffic shaping-over-Frame Relay feature, the central router would note that the BECN was received, but would not slow its transmission rate.
  3. At this point, packets begin dropping within the switch that is encountering the congestion. As previously discussed, this environment can cause problems, particularly for latency-sensitive protocols such as SNA.

With the problem in this given process, the solution is to enable the central router to dynamically fluctuate the rate at which it sends packets, depending on the BECNs that it receives. For example, if the central router begins receiving many BECNs, it will reduce the packet transmit rate. As the BECNs become intermittent, the router will increase the packet transmit rate.

Perform the following steps to configure traffic shaping-over-Frame Relay BECN support:

  1. Define a map class and enter map class configuration mode, as previously discussed.
  2. Make sure that BECN support is enabled:

Router(config-map-class)#frame-relay adaptive-shaping becn

BECN support is enabled by default. When enabled, BECNs received from the network on this VC are used to further regulate the output rate on the VC. As the frequency of BECNs increases, the output rate is steadily reduced from peak to average (equivalent of CIR). As congestion eases in the network and the frequency of BECNs decreases, the output rate is allowed to increase gradually to its configured peak.

  1. Enable both traffic shaping and per-VC queuing for all VCs (PVCs and SVCs) on a Frame Relay interface:

Router(config-if)#frame-relay traffic-shaping

For VCs, where no specific traffic shaping or queuing parameters are specified, a set of default values is used.

  1. Associate a map class with an interface or subinterface, as previously discussed:

Router(config-if)#frame-relay class name

  1. (Optional) Apply the map class to a specific DLCI for which a frame-relay map statement exists, as previously discussed:

Router(config-map-class)#frame-relay interface-dlci
dlci [broadcast] [ietf | cisco]

 
Lab Activity    
  Your main office in Atlanta has a high-speed T1 Frame Relay connection. There is a bottleneck that exists at the other end of the PVC that connects to Orlando. This is due to the fact that Orlando only has a 56K Frame Relay connection.