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Webcast-Catalyst9k
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auto qos voip trust effect on 2960G (IOS 12.2(46)SE)

I use the interface config command "auto qos voip trust" on Gbps interfaces where are connected non CISCO ToIP phone+attached PC;

we also configure a policy-map to set mission-critical data flows to PHB AF31 (=DSCP 26 = COS 3) and associate this policy on the same interfaces ;

the result of this config in terms of bandwith sharing on these interfaces is based on the following "srr bandwidth" command automatically generated by the "auto qos voip trust" command :

interface g0/1
srr-queue bandwidth share 10 10 60 20
queue-set 2
priority-queue out
mls qos trust cos
auto qos voip trust
and the COS to Queue/Threshold mapping is as following (also resulting from "auto qos voip trust" command) :

   Cos-outputq-threshold map:

              cos:  0   1   2   3   4   5   6   7

              ------------------------------------

  queue-threshold: 4-3 4-2 3-3 2-3 3-3 1-3 2-3 2-3

So, for the 2 interesting flows here (ToIP flow marked by phones with DSCP 46/COS 5, and Mission-critical flows marked by the policy-map to DSCP 26, COS 3), the shared bandwith will give this result :

1) the COS 5 ToIP frames, are mapped to queue 1, which is defined as a Priority Queue thanks to option "priority-queue out" on the interface : this is fine

2) the COS 3 Mission-Critical frames are mapped to queue 2, which has a shared bandwidth of 10% of the interface,

while COS 2 frames (supposedly less critical) would be mapped to queue 3 which has a shared 60% of the interface, and COS 1 & 1 (supposedly not critical at all) would be mapped to queue 4 which has a shared bandwidth of 20% of the interface

Question :

why does the "auto qos voip trust" generate bandwidth values which don't increase with the COS value but seem to do the reverse in case of COS0,1 and 2 as shown above ?

is this "auto qos voip trust" command really useful ?

same problem would be present in case of uplinks interface (with no policy-map added)

thanks in advance

Everyone's tags (3)
1 REPLY
New Member

auto qos voip trust effect on 2960G (IOS 12.2(46)SE)

I forgot to mention that the commands generated by "auto qos voip trust" command are shown in the QOS Configuration Guide here :

http://www.cisco.com/en/US/docs/switches/lan/catalyst2960/software/release/12.2_46_se/configuration/guide/swqos.html

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