CCIE Pursuit Blog

August 20, 2008

Lab Tip: Finding Default WRED Values

Here’s a quick and dirty method to find default WRED values so that if a task asks you to reference the defaults (i.e. “Make the maximum threshold twice the default”) you will be able to quickly find the default values without searching the Cisco documentation.

First turn WRED on for an interface:

r1(config)#int fa0/1
r1(config-if)#random-detect

Now you can issue the “show queueing interface f0/1” command:

r1#show queueing interface f0/1
Interface FastEthernet0/1 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

This shows the default WRED settings for each IP precedence class.  The default values for IP precedence 3 are:

r1(config-if)#random-detect precedence 3 26 40 10

Where 3 = IP Precedence; 26 = Minimum Threshold; 40 = maximum threshold; and 10 = mark probability denominator

This is good to know because you may be asked to change one of these variables.  To change one of these variables you still need to enter in values for the other variables so you need to know the default values if you are not tasked with changing them.  You could look up the defaults in the DOC, but this is faster.

What if you want the DSCP defaults instead?  One more line will yield those for you:

r1(config)#int fa0/1
r1(config-if)#random-detect dscp-based

r1(config-if)#do sh queueing int f0/1
Interface FastEthernet0/1 queueing strategy: random early detection (WRED)
    Random-detect not active on the dialer
    Exp-weight-constant: 9 (1/512)
    Mean queue depth: 0

   dscp          Random drop      Tail drop    Minimum Maximum  Mark
                  pkts/bytes       pkts/bytes    thresh  thresh  prob
   af11      0/0              0/0           33      40  1/10
   af12      0/0              0/0           28      40  1/10
   af13      0/0              0/0           24      40  1/10
   af21      0/0              0/0           33      40  1/10
   af22      0/0              0/0           28      40  1/10
   af23      0/0              0/0           24      40  1/10
   af31      0/0              0/0           33      40  1/10
   af32      0/0              0/0           28      40  1/10
   af33      0/0              0/0           24      40  1/10
   af41      0/0              0/0           33      40  1/10
   af42      0/0              0/0           28      40  1/10
   af43      0/0              0/0           24      40  1/10
    cs1      0/0              0/0           22      40  1/10
    cs2      0/0              0/0           24      40  1/10
    cs3      0/0              0/0           26      40  1/10
    cs4      0/0              0/0           28      40  1/10
    cs5      0/0              0/0           31      40  1/10
    cs6      0/0              0/0           33      40  1/10
    cs7      0/0              0/0           35      40  1/10
     ef      0/0              0/0           37      40  1/10
   rsvp      0/0              0/0           37      40  1/10
default       0/0              0/0           20      40  1/10

Remember to remove WRED (or DSCP) if you’re not using it on that interface:

r1(config)#int fa0/1
r1(config-if)#no random-detect

r1(config-if)#do sh queueing int f0/1
Interface FastEthernet0/1 queueing strategy: none

If you’re given an IP Precedence name like “flash-override” instead of the IP Precedence value (4 in this case) then use this tip:

Lab Tip: Remembering IP Precedence Values

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