Relationship between 3550 egress queues and bandwidth

From: simon hart (simon@harttel.com)
Date: Mon Sep 05 2005 - 18:10:25 GMT-3

Dear all,

I am a little confused with reference to qos on the 3550. This relates to
the interaction between applying an outbound service policy to an interface
and the WRR queues.

So for instance I create a policy such as

 Policy Map QOS
  class PREC4
      Bandwidth 30(%) Max Thresh 64 (packets)
  class PREC5
      Bandwidth 20 (%) Max Thresh 64 (packets)

applied to an access interface

interface FastEthernet0/2
 switchport access vlan 263
 switchport mode access
 no ip address
 service-policy output QOS

The policy map dictates that ip precedence 4 traffic will guarantee 10% of
bandwidth and ip precedence 5 traffic will guarantee 20% bandwidth,

However if I look at the wrr-queue

FastEthernet0/2
Egress expedite queue: dis
wrr bandwidth weights:
qid-weights
 1 - 25
 2 - 25
 3 - 25
 4 - 25
Cos-queue map:
cos-qid
 0 - 1
 1 - 1
 2 - 2
 3 - 2
 4 - 3
 5 - 3
 6 - 4
 7 - 4

Assuming that all the mutation maps are default, then ip prec 4 and ip prec
5 will be placed in qid 3, and thus will have 25% of the bandwidth.

So under the policy map there is a total guarantee of 50% but on the
interface wrr-queue there is only 25% assigned. Which takes precedence over
which, or am missing something

Thanks in advance

Simon 

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