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.

jac
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