Frame Relay Notes - DE, FECN, and BECN
- 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.
http://www.sinclair.org.au/keith/networking/frame_relay.html
—
Corrections requested.