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Frame-relay Discard Eligibility Bit (DE)

What is the DE bit ?

The DE bit is used to indicate that the frame has a lower importance than other frames and should be dropped first if the network experienced congestion.

How to set the DE bit?

We can use a simple network to test our configuration. R1 is connected via frame-relay to R2

Method1: Frame-relay de-list command
R1 Configuration

R1(config)#access-list 1 permit 1.1.1.1

!-- packets sourced from 1.1.1.1 with size > 500 will have DE bit set

R1(config)#frame-relay de-list 1 protocol ip list 1
R1(config)#frame-relay de-list 1 protocol ip gt 500

!-- assign the DE list to the DLCI

R1(config-if)#int s1/0
R1(config-if)#frame-relay de-group 1 102

Verification and troubleshooting:

!-- Before sending any frames with DE bit set

R1#show frame-relay pvc 102

PVC Statistics for interface Serial1/0 (Frame Relay DTE)

DLCI = 102, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial1/0

  input pkts 14            output pkts 13           in bytes 1282
  out bytes 1226           dropped pkts 0           in pkts dropped 0
  out pkts dropped 0                out bytes dropped 0
  in FECN pkts 0           in BECN pkts 0           out FECN pkts 0
  out BECN pkts 0          in DE pkts 0             out DE pkts 0
  out bcast pkts 3         out bcast bytes 186
  5 minute input rate 0 bits/sec, 0 packets/sec
  5 minute output rate 0 bits/sec, 0 packets/sec
  pvc create time 00:02:05, last time pvc status changed 00:00:55

!-- ping source is 1.1.1.1 with size greater than 500 as configured in the de-list

R1#ping 192.168.123.2 source lo 1 size 1000

Type escape sequence to abort.
Sending 5, 1000-byte ICMP Echos to 192.168.123.2, timeout is 2 seconds:
Packet sent with a source address of 1.1.1.1
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 44/107/184 ms
R1#show frame-relay pvc 102                

PVC Statistics for interface Serial1/0 (Frame Relay DTE)

DLCI = 102, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial1/0

  input pkts 25            output pkts 24           in bytes 6758
  out bytes 6702           dropped pkts 0           in pkts dropped 0
  out pkts dropped 0                out bytes dropped 0
  in FECN pkts 0           in BECN pkts 0           out FECN pkts 0
  out BECN pkts 0          in DE pkts 0             out DE pkts 5
  out bcast pkts 9         out bcast bytes 642
  5 minute input rate 1000 bits/sec, 1 packets/sec
  5 minute output rate 2000 bits/sec, 0 packets/sec
  pvc create time 00:05:05, last time pvc status changed 00:03:55

!-- R2 output is showing DE packets received from R1

R2#sh frame-relay pvc 201

PVC Statistics for interface Serial1/0 (Frame Relay DTE)

DLCI = 201, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial1/0

  input pkts 27            output pkts 27           in bytes 6930
  out bytes 6952           dropped pkts 0           in pkts dropped 0
  out pkts dropped 0                out bytes dropped 0
  in FECN pkts 0           in BECN pkts 0           out FECN pkts 0
  out BECN pkts 0          in DE pkts 5             out DE pkts 0
  out bcast pkts 12        out bcast bytes 892
  5 minute input rate 0 bits/sec, 0 packets/sec
  5 minute output rate 0 bits/sec, 0 packets/sec
  pvc create time 00:06:13, last time pvc status changed 00:06:13

Method2: Using MQC

R1 Configuration

!-- match packets sourced from 1.1.1.1 with size > 500

R1(config)#class-map match-all FRDE
R1(config-cmap)#match access-group 1
R1(config-cmap)#match packet length min 500

!-- Set DE bit for matched frames

R1(config)#policy-map SETDE
R1(config-pmap)#class FRDE
R1(config-pmap-c)#set fr-de

!-- Assign the Policy map to the interface

R1(config)#int s1/0
R1(config-if)#service-policy output SETDE

Verification and troubleshooting:

!-- Before sending any DE frames

R1#sh fram pvc 102

PVC Statistics for interface Serial1/0 (Frame Relay DTE)

DLCI = 102, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial1/0

  input pkts 4             output pkts 2            in bytes 222
  out bytes 152            dropped pkts 0           in pkts dropped 0
  out pkts dropped 0                out bytes dropped 0
  in FECN pkts 0           in BECN pkts 0           out FECN pkts 0
  out BECN pkts 0          in DE pkts 0             out DE pkts 0  
  out bcast pkts 2         out bcast bytes 152
  5 minute input rate 0 bits/sec, 0 packets/sec
  5 minute output rate 0 bits/sec, 0 packets/sec
  pvc create time 00:00:58, last time pvc status changed 00:00:58

!-- Generate frames with DE bit set

R1#ping 192.168.123.2 source lo 1 size 1000

Type escape sequence to abort.
Sending 5, 1000-byte ICMP Echos to 192.168.123.2, timeout is 2 seconds:
Packet sent with a source address of 1.1.1.1
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 28/148/244 ms
R1#sh fram pvc 102                         

PVC Statistics for interface Serial1/0 (Frame Relay DTE)

DLCI = 102, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial1/0

  input pkts 17            output pkts 15           in bytes 5990
  out bytes 5920           dropped pkts 0           in pkts dropped 0
  out pkts dropped 0                out bytes dropped 0
  in FECN pkts 0           in BECN pkts 0           out FECN pkts 0
  out BECN pkts 0          in DE pkts 0             out DE pkts 5
  out bcast pkts 5         out bcast bytes 380
  5 minute input rate 1000 bits/sec, 1 packets/sec
  5 minute output rate 1000 bits/sec, 1 packets/sec
  pvc create time 00:02:26, last time pvc status changed 00:02:26

R2#sh frame-relay pvc 201

PVC Statistics for interface Serial1/0 (Frame Relay DTE)

DLCI = 201, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial1/0

  input pkts 6             output pkts 6            in bytes 5096
  out bytes 5096           dropped pkts 0           in pkts dropped 0
  out pkts dropped 0                out bytes dropped 0
  in FECN pkts 0           in BECN pkts 0           out FECN pkts 0
  out BECN pkts 0          in DE pkts 5             out DE pkts 0
  out bcast pkts 1         out bcast bytes 76
  5 minute input rate 2000 bits/sec, 0 packets/sec
  5 minute output rate 2000 bits/sec, 0 packets/sec
  pvc create time 00:18:39, last time pvc status changed 00:01:03

I hope I have been informative in this article, see you in other posts.

2 comments

  1. Hi,

    Thanks for sharing.

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