Frame Relay Congestion Control

Frame Relay switch detects congestion on output queue and informs:

- The receiver by setting the FECN bit on forwarded frames

- The source by setting the BECN bit on frames going in the opposite direction

A Frame Relay switch can explicitly report congestion in two directions: forward and backward.

When a frame queue inside a switch is congested, the switch will generate congestion signals based on the FECN and BECN bits. If congestion occurs in a queue transmitting toward the main receiver of traffic, FECN signals are sent to the receiving Layer 2 endpoint and BECN signals are sent to the sending Layer 2 endpoint.

FECN and BECN bits are not sent as separate frames, but use bits in the header of data frames.

This topic describes how class-based shaping can adapt dynamically to available Frame Relay bandwidth by integrating BECN signals.

Class-based shaping is able to respond to Layer 2 congestion in the Frame Relay network by reducing its shaping rate to three-quarters of the current rate, until the Layer 2 Frame Relay network recovers from congestion. When BECN flags are no longer received, the rate is slowly increased to the original shaping rate.

This topic describes the FECN-to-BECN propagation mechanism.

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