How to configure Queuing on Cisco 3560 and 3750 switch

[lagapidis (Lazarus Agapidis)]

Hello Sze Jie K

I’ll attempt to answer your questions below:

@szejiekoh

mls qos cos x
Q1) When mls cos is turned on and all the ports are untrusted, does the switch reset the COS value for all frames to whatever value set in the “mls qos cos” command ?

Yes. When a port is set to untrusted (by default) it does not trust the CoS values of the incoming frames, so those are reset to the value set in the command.

@szejiekoh

Q2) When mls cos is turn on and all the ports are trusted, the switch will only use the value in “mls qos cos” command to set for untagged frames right ?

Yes. On a trunk port, if you have the mls qos cos command on a trusted port, then the CoS values will be changed only for untagged frames. The CoS values of tagged frames remain unchanged.

@szejiekoh

mls qos trust cos
Q3) When a policy-map + mls qos cos trust is set on the same port, which will have priority ? Assuming the policy-map will set CoS value and the incoming frame also already has a CoS value set.

The trust command in a policy map allows you to set the trust state only for the traffic defined within that particular class. mls qos trust applies to ALL traffic entering the interface. Any traffic conforming to the policy map will have priority. Traffic not conforming will have the more general mls qos trust command applied to it.

@szejiekoh

priority-queue out
Q4) When egress expedite queue is turn on, does the shaped bandwidth defined on queue1 still applies ?

According to Cisco:

All four queues participate in the SRR unless the expedite queue is enabled, in which case the first bandwidth weight is ignored and is not used in the ratio calculation. The expedite queue is a priority queue, and it is serviced until empty before the other queues are serviced. You enable the expedite queue by using the priority-queue out interface configuration command.

This was obtained from the following documentation, page 29-17:

swqos.pdf

@szejiekoh

Q5) Can we mapped the same CoS to more than 1 egress queue ?
CoS 3 to both egress q2 t2 and q3 t1, how will the switch handle the distribution if q2 and q3 has the same weight ?
If the switch send the frame with CoS 3 to q2t2 and q2t2 threshold is already hit, will the switch send the frame to q3t1 or it will simply drop it ?

You can set the same CoS value to be mapped to multiple queues. However, for the rest of your questions, this might need to be labbed up, unless @ReneMolenaar has a ready answer…

I hope this has been helpful!

Laz