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Cisco IOS Cookbook, 2nd Edition by Ian Brown, Kevin Dooley

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Controlling Congestion with WRED

Problem

You want to control congestion on an interface before it becomes a problem.

Solution

The syntax for configuring WRED changed with the introduction of class-based QoS. The old method defined WRED across an entire interface:

Router#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router(config)#interface HSSI0/0
Router(config-if)#random-detect
Router(config-if)#random-detect precedence 0 10 20 10
Router(config-if)#random-detect precedence 1 12 20 10
Router(config-if)#random-detect precedence 2 15 25 15
Router(config-if)#random-detect precedence 3 18 25 15
Router(config-if)#random-detect precedence 4 20 30 20
Router(config-if)#random-detect precedence 5 22 30 20
Router(config-if)#random-detect precedence 6 30 40 25
Router(config-if)#random-detect precedence 7 40 50 100
Router(config-if)#random-detect precedence RSVP 45 50 100
Router(config-if)#exit
Router(config)#end
Router#

The new configuration method uses the same syntax as CBWFQ:

Router#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router(config)#class-map Prec5
Router(config-cmap)#description Critical
Router(config-cmap)#match ip precedence 5
Router(config-cmap)#exit
Router(config)#policy-map cb_wred
Router(config-pmap)#class Prec5
Router(config-pmap-c)#random-detect dscp-based
Router(config-pmap-c)#exit
Router(config-pmap)#class class-default
Router(config-pmap-c)#fair-queue 512
Router(config-pmap-c)#queue-limit 96  Router(config-pmap-c)# ...

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