Cisco Catalyst 3750 Switch QoS Issue for MPLS Remote Site


We have a network of Avaya VOIP handsets powered by Cisco Catalyst 3750 PoE switches. Our Avaya PBX (CLAN's, MedPro's etc) and VOIP handsets all tag voice and signalling packets with DSCP value of 46 as normal and all our other data traffic is marked as default DSCP = 0.

We have never experienced bandwidth or capacity issues on our Cisco Catalyst's to date and have never needed QoS to be fully configured for traffic shaping, policing, classification, marking bandwidth throttling etc.

There seems to be minimal QoS configuration on our switches, it just looks to be enabled but not configured to offer any special preference to voice over data.

The output of the "show mls qos" command is below:

QoS is enabled
QoS ip packet dscp rewrite is enabled

We now are about to connect a remote site through an MPLS site-to-site connection and need to configure QoS correctly so that voice and signalling packets are given appropriate preference through the service providers MPLS cloud.

I have run SPAN on a switch port with a VOIP phone attached and it looks that traffic leaving the phone and entering the switch port is being marked by the phone as DSCP = 46 [expedited forwarding] but when it reached the switch the ingress interface is stripping off the DSCP value and resetting it back to the default of 0. This seems also to be the case with any marking done by the PBX (CLAN's, MedPro's etc).

I now need the switch to preserve the the DSCP field and not rewrite the DSCP value back to 0.

How can I do this without affecting the performance of our switch and still mark the voice packets so that when it enters the MPLS router is maintains the DSCP value of 46? I'm looking for the simplest configuration.

I understand I could fully configure QoS on the Catalyst 3750's and I understand the benefits but for the time being I really just need to preserve the DSCP markings for the MPLS.



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Don JohnstonInstructorCommented:
If you want to preserve the QOS marking, you need to "trust" the interface.

mls qos trust dscp

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