FECN and BECN

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Contents

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12.4T Configuration Guides -> WAN -> Cisco IOS Wide-Area Networking Configuration Guide, Release 12.4T -> Frame Relay -> Adaptive Frame Relay Traffic Shaping for Interface Congestion

Introduction

FECN (Forward Explicit Network Congestion) and BECN (Backward Explicit Network Congestion) are bits used to indicate that network congestion has occurred.

FECNs are used by frame relay switches to indicate to a node that congestion is occurring towards that node. A node should then set the BECN flag on return traffic to indicate to the original source to slow down their transmission rate.

Example 1

Adaptive shaping based on BECNs.

map-class frame-relay frts
 frame-relay cir 1500
 frame-relay mincir 1000
 frame-relay adaptive-shaping becn
!
interface Serial0/0
 ip address 172.16.12.6 255.255.255.0
 encapsulation frame-relay
 no fair-queue
 frame-relay class frts
 frame-relay traffic-shaping
 frame-relay map ip 172.16.12.6 602 broadcast
 frame-relay map ip 172.16.12.2 602 broadcast
 no frame-relay inverse-arp

Verification

Show frame pvc <<pvc>>

Example demonstrating adaptive traffic shaping based on receipt of BECNs.

R6#show frame pvc 602

PVC Statistics for interface Serial0/0 (Frame Relay DTE)

DLCI = 602, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial0/0

  input pkts 68436         output pkts 5882         in bytes 6021541
  out bytes 513494         dropped pkts 54869       in pkts dropped 0
  out pkts dropped 54869            out bytes dropped 5267528
  late-dropped out pkts 54869       late-dropped out bytes 5267528
  in FECN pkts 0           in BECN pkts 0           out FECN pkts 0
  out BECN pkts 0          in DE pkts 0             out DE pkts 12
  out bcast pkts 1         out bcast bytes 34
  5 minute input rate 29000 bits/sec, 41 packets/sec
  5 minute output rate 0 bits/sec, 0 packets/sec
  Shaping adapts to BECN
  pvc create time 22:31:28, last time pvc status changed 17:40:11
  cir 1500      bc 1500      be 0         byte limit 23     interval 125
  mincir 1000      byte increment 23    Adaptive Shaping BECN
  pkts 5161      bytes 494952    pkts delayed 5143      bytes delayed 493752
  shaping inactive
  traffic shaping drops 0
  Queueing strategy: fifo
  Output queue 0/40, 54869 drop, 5143 dequeued

See Also

  1. Cisco on Frame Relay traffic shaping
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