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.

Aaron Conaway

I shake my head around sometimes and see what falls out. That's what lands on these pages. If you have any questions, the best way to contact me is through Twitter at @aconaway.

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