Trusting Both CoS and DSCP on Cisco 3750

Unanswered Question
Sep 13th, 2010

We are currently running IOS from 12.2(35) to 12.2(50) on either a Cisco Catalyst 3750 or 3750v2 switches.

Is it possible to trust both CoS and DSCP on the switch ports at the same time as seen in a example

configuration below. The reason we would like to trust both, CoS for backward compatiablity for older

equipment and DSCP for new equipment and future path to transition to IPv6 that uses DSCP.

description Workstation/Phone
switchport access vlan 2096
switchport mode access
switchport voice vlan 2097
priority-queue out
mls qos trust device cisco-phone
mls qos trust cos
mls qos trust dscp
ip arp inspection limit 60
no mdix auto
spanning-tree portfast
spanning-tree bpduguard enable

I have this problem too.
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jonathanaxford Tue, 09/14/2010 - 00:45

Hi Jeff,

I am not sure about trusting both values on the same interface - it is probably better to do this on a case-by-case basis? Mark those ports which require CoS to be trusted as such and then those that require DSCP with that.

I am pretty sure that both commands are exclusive of each other - you can either trust one or the other, not both at the same time. This allows the switch to determine which queue to place the traffic in, or whether to manipulate the marking in anyway based on one of the values. You can use Cos-to-DSCP maps to help mark the CoS traffic with the correct DSCP values and vica-versa, ensuring that you have valid QoS marking poulicy all the way through the network, regardless of whether your equipment supports Cos or DSCP (or both).

Does this help at all?

Many thanks


Lei Tian Tue, 09/14/2010 - 04:14

Hi Jeff,

As Jonathan mentioned, you can't trust both COS/DSCP on one port. The later entered command will over write the previous one.

One solution of trusting both is to disable DSCP rewrite on the switch, and trust COS on the trunk port. That way, incoming frame with COS will be trusted, and the origin DSCP bits will be reserved. On the access port, just trust DSCP.


Lei Tian


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