8.4 Frame Relay Traffic Shaping
8.4.3 Using traffic shaping over Frame Relay
The traffic shaping over Frame Relay feature can be used in the following typical situations:
  • When you have a Frame Relay network topology that consists of a high-speed (T1 line speed or greater) connection at the central site and low-speed (56 Kbps or less) connections at the branch sites
  • When you have a Frame Relay network that is constructed with many VCs to different locations on a single physical line into the network
  • If you notice that your Frame Relay connections occasionally get congested
  • When you have several different types of traffic (such as IP, Systems Network Architecture [SNA], or IPX) to transmit on the same Frame Relay VC, and want to ensure that the different traffic types receive a certain amount of bandwidth.

When you have a Frame Relay network topology that consists of a high-speed (T1 line speed or greater) connection at the central site and low-speed (56 Kbps or less) connections at the branch sites, as shown in the Figure, you can use the traffic shaping over Frame Relay feature. Because of the speed mismatch, a bottleneck often exists for traffic on a VC when the central site tries to communicate with the branch site. This bottleneck results in poor response times for traffic, such as SNA or interactive Telnet when it is stuck behind a large File Transfer Protocol (FTP) packet on the low-speed line. Packets get dropped at the bottleneck, resulting in lost SNA sessions and possibly causing the central site to retransmit unacknowledged packets - making the congestion problem worse. The rate enforcement capability of the traffic shaping feature can be used to limit the rate at which data is sent on the VC at the central site. Rate enforcement can also be used in conjunction with the existing DLCI prioritization feature to further improve performance in this situation.

When you have a Frame Relay network that is constructed with many VCs to different locations on a single physical line into the network, these VCs send traffic as fast as the physical line speed allows. The rate enforcement capability enables you to control the transmission speed used by the router by other criteria, such as the CIR or EIR. The rate enforcement feature can preallocate the bandwidth that each VC receives on the physical line into the network, effectively creating a virtual time-division multiplexing network.

If you notice that your Frame Relay connections occasionally get congested, you may want the router to throttle traffic instead of sending it in to the network. Throttling the traffic may help prevent packet loss in the network. The BECN-based throttling capability that is provided with the traffic shaping feature allows you to have the router dynamically throttle traffic, based on receiving BECN-tagged packets from the network. This throttling holds packets in the router buffers to reduce the data flow from the router into the Frame Relay network. The throttling is done on a per-VC basis, and the rate is dynamically increased as fewer BECNs are received.

Often you may have several different types of traffic, such as IP, SNA, or IPX, to transmit on the same Frame Relay VC, and want to ensure that the different traffic types receive a certain amount of bandwidth. Using CQ (custom queuing), with the per-VC queuing and rate enforcement capabilities, enables you to configure VCs to perform this task. Prior to Cisco IOS Release 11.2, CQ was defined only at the interface level. Now it can be defined at the VC level.