QOS COS

mikegrous · ‎03-19-2009

I want to make sure i have this correct:

lets say a 3560 switch here:

mls qos is enabled, nothing else.

packet comes in at cos 5.. it will get rewritten to 0. The reason it is 0 is because the mls qos cos 0 command which is default. If i change this command to mls qos cos 1 then all packets would be cos 1. Correct?

Adding to above scenario. If i do mls qos trust cos.. all cos packets coming in at 5 will remain at 5. All packets coming in unmarked will get changed to cos 1. Is this correct?

Building on that. If i now do mls qos override. ALL Packets will get changed to cos 1.

Am i wrong in any of my above understanding?

drolemc · ‎03-25-2009

Queueing evaluates the QoS label and the corresponding DSCP or CoS value to select into which of the two ingress queues to place a packet.

I hope below link explains the mls qos cos & mls qos trust cos.

http://www.cisco.com/en/US/docs/switches/lan/catalyst3560/software/release/12.2_25_see/configuration/guide/swqos.html

Jon Marshall · ‎03-25-2009

Mike

"The reason it is 0 is because the mls qos cos 0 command which is default"

Correct.

"If i change this command to mls qos cos 1 then all packets would be cos 1"

Yes, correct again. By using this command you have changed default value of a port to be Cos 1. So any untagged packets will have their value set to Cos 1.

"Building on that. If i now do mls qos override. ALL Packets will get changed to cos 1"

Yes, correct again.

Your understanding is spot on.

Jon

greg.loria · ‎04-21-2009

Hi Jon, are these the same with dscp?

We are having trouble with our present setup:

We have two 4500 switches, A and B that are connected by a trunk link. The packet leaves Switch A with a mark of QoS 5 but the value returns to default after passing through the link. Why do you think this issue occurs even though qos trust dscp is configured?

Thanks.