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.
Was this article helpful?