Frame Relay Notes - DE, FECN, and BECN

Thu, 23 Jun 2011 00:00:00 +0000

All are part of the frame relay congestion management suite.
Frame relay switches monitor links for CIR or oversubscription congestion on links.
- If the VC has a CIR of 256k, the switch knows there is congestion if the customer is sending more than 256k down that VC.
Discard Eligible
- Flag in the LAPF header
  - Marks a frame as eligible to be dropped in case of congestion
  - Marked via the MQC
Forward Explicit Congestion Notification
- Flag in the LAPF header
- Set by the switch when the frame is about to enter a link with congestion on a VC
  - Congestion in one direction
  - FECNs are set when the frame is going into the congestion.
- Receiving router can see that there was congestion on the way.
- FECNs can be used to activate adaptive shaping via FRTS.
- Plain English: If Router B receives a frame with the FECN flag set, that means that there is congestion on the path from Router A to this Router B, and that Router B should expect delays.
Backward Explicit Congestion Notification
- Flag in the LAPF header
- Set by the switch when a frame has just left the link with congestion
  - Congestion is the opposite direction.
  - BECNs are set when the frame has just left a link that has congestion on it.
- Notifies the original sending router that there is congestion along that VC.
- Plain English: If Router A receives a frame with the BECN flag set, that means that there is congestion from Router A towards Router B and that the sending host should calm down a little bit.

Becn on Aaron's Worthless Words