DSCP and Cos in switch 3560

[wfqk]

Hi Can you take a look at the below. Why there are two marking DSCP and Cos in one port? Any one can explain it ? Thank you

SW(config-if)#do sh mls qos int f0/11
FastEthernet0/11
trust state: trust dscp
trust mode: trust dscp
trust enabled flag: ena
COS override: dis
default COS: 5
DSCP Mutation Map: Default DSCP Mutation Map
Trust device: cisco-phone
qos mode: port-based

[Seb Rupik]

Hi there,

The default COS value is used if the interface is COS trusted and the packets are untagged (adding the override command will adjust the COS vlaues on all incoming packets).

However in your case fa0/11 is configured to trust the DSCP values of incoming packets, so the default COS value can be ignored.

cheers,

Seb.

[wfqk]

Thank you for your reply. 

You mentioned: "and the packets are untagged (adding the override command will adjust the COS vlaues on all incoming packets)."

what is untagged? you mean "unmarked"?

And after using "override", both trust state and trust mode change to "not trusted" though default cost is 5. so do you think the switch is still trust boundary?

Please see the below:

SW(config-if)#do sh mls qos int f0/11
FastEthernet0/11
trust state: not trusted
trust mode: not trusted
trust enabled flag: ena
COS override: ena
default COS: 5
DSCP Mutation Map: Default DSCP Mutation Map
Trust device: none
qos mode: port-based

Thank you

[Seb Rupik]

Hi there,

Yes the correct term should be untagged.

When you issue the override command it takes preccedence over any trust dscp/ cos commands you have configured on the interface. With the override enables any packet tagged/ untagged will have its COS value changed to the specified default COS value. The default COS-to-DCSP map will then be applied to the packet.

With the override command, since you are modifying all incoming QoS tags, then yes the interface is still a trust boundary.

cheers,

Seb.