Frame Relay Congestion Control

Remember back to our talk about CIR? From that, it should be obvious that the lower your CIR is set, the greater the risk is that your data will become toast. This can be easily avoided if you have just one key piece of information—when and when not to transmit that huge burst! This begs the question, Is there any way for us to find out when our telco’s shared infrastructure is free and clear and when it’s crammed and jammed? Also, if there is a way to spy this out, how do you do it? Well, that’s exactly what I’m going to talk about next—how the Frame Relay switch notifies the DTE of congestion problems—and address those very important questions.

Here are the three congestion bits and their meanings:

Discard Eligibility (DE) As you know, when you burst (transmit packets beyond the CIR of a PVC), any packets exceeding the CIR are eligible to be discarded if the provider’s network is congested at the time. Because of this, the excessive bits are marked with a Discard Eligibility (DE) bit in the Frame Relay header. And if the provider’s network happens to be congested, the Frame Relay switch will discard the packets with the first DE bit set. So if your bandwidth is configured with a CIR of zero, the DE will always be on.

Forward Explicit Congestion Notification (FECN) When the Frame Relay network recognizes congestion in the cloud, the switch will set the Forward Explicit Congestion Notification (FECN) bit to 1 in a Frame Relay packet header. This will indicate to the destination DTE that the path the frame just traversed is congested.

Backward Explicit Congestion Notification (BECN) When the switch detects congestion in the Frame Relay network, it’ll set the Backward Explicit Congestion Notification (BECN) bit in a Frame Relay frame that’s destined for the source router. This notifies the router that congestion is ahead. But Cisco routers won’t take action on this congestion information unless you tell them to.