WRED - Part I precedence based

The TCP algorithm by default will cause TCP senders to back off when packet loss occurs. One of the main problems with this is that all senders will back off at the same time and slowly build up the transmission rate until packet loss occurs again. Much like cars on a three lane motorway this causes the 'concertina effect' where all traffic backs off and speeds up at the same rate.

The aim of WRED or Weighted Random Early Detect is to get around this problem by intelligently getting senders to back off at different times based on the configured weightings.

To enable the default WRED algorithim simply requires the config-if#random-detect parameter to be entered on the required interface.

The default settings can be viewed with the show queueing random-detect command.

R1#s queueing random-detect
Current random-detect configuration:
FastEthernet0/0
Queueing strategy: random early detection (WRED)
Random-detect not active on the dialer
Exp-weight-constant: 9 (1/512)
Mean queue depth: 0

class Random drop Tail drop Minimum Maximum Mark
pkts/bytes pkts/bytes thresh thresh prob
0 0/0 0/0 20 40 1/10
1 0/0 0/0 22 40 1/10
2 0/0 0/0 24 40 1/10
3 0/0 0/0 26 40 1/10
4 0/0 0/0 28 40 1/10
5 0/0 0/0 31 40 1/10
6 0/0 0/0 33 40 1/10
7 0/0 0/0 35 40 1/10
rsvp 0/0 0/0 37 40 1/10


Of note in the above output:-

class 0 -7 : denotes the IP precedence value.
tail drop pkts Min: denotes the interface queue size at which packets will start to be dropped.
tail drop pkts Max: denotes the interface queue size at which all packets will be dropped.
Mark Prob: denotes the fraction of packets that will be dropped once the tail drop pkts Min has been reached. The default being 1 in 10.

If there was a lab requirement to ensure packets with an ip precedence of 0 (routine) were to be subject to tail drop when the interface queue size reached 10 (as opposed to the default of 22) this can be achieved as follows:

int fa0/0
randon-detect precedence 0 10 40 10

Once entered the new parameter settings can be viewed by using the show queueing random-detect command again.

R1#s queueing random-detect
Current random-detect configuration:
FastEthernet0/0
Queueing strategy: random early detection (WRED)
Random-detect not active on the dialer
Exp-weight-constant: 9 (1/512)
Mean queue depth: 0

class Random drop Tail drop Minimum Maximum Mark
pkts/bytes pkts/bytes thresh thresh prob
0 0/0 0/0 10 40 1/10
1 0/0 0/0 22 40 1/10
2 0/0 0/0 24 40 1/10
3 0/0 0/0 26 40 1/10
4 0/0 0/0 28 40 1/10
5 0/0 0/0 31 40 1/10
6 0/0 0/0 33 40 1/10
7 0/0 0/0 35 40 1/10
rsvp 0/0 0/0 37 40 1/10

WRED can also be enabled based on DSCP settings and also based on configured traffic classed. Food for another post.