QOS trusts

Unanswered Question
Nov 20th, 2008

Hi all, when a packet travels from one switch to another , and the packet say from switch A is marked with say cos 5, if this enters another switch with no trust, wha will happen, will it keep the marking,but the switch will not put it in the queue ?

I have this problem too.
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Sebastian Helmer Thu, 11/20/2008 - 01:38


when a packet will receive a port with cos 5 the port must trussed the QoS value. When not the cos value will reset to "0" and the packet forwarded without QoS benefit in a normal configurationt (cos mapping)

mbroberson1 Thu, 11/20/2008 - 07:41


As s.helmer mentioned it will overwrite/rewrite the TOS bits with a cos of 0. If you wish to keep the markings you can place "mls qos trust cos" on the trunk links.



carl_townshend Thu, 11/20/2008 - 07:49

Hi there, thanks for that, surely the switch must have sme processor overhead for having to do this?

What happens about the DSCP values? do they remain intact?

Also can you confirm that when you have a cos to dscp mapping in a switch, if the switch sees either cos or dscp marking, would it change the other? ie if it sees cos 5 would it change the dcsp accordingly and vice versa ?

Sebastian Helmer Thu, 11/20/2008 - 11:17

I'm not sure about the CPU I think it will take very much CPU.

Yes it depends on the settings. If you configure nothing all markings will reset/remark to "0"....



pls. vote


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