I have a question regarding a branch office design, and a Data Center design.
2960/3560/3750 L2 switches, The VOICE VLAN could be on router/or SVI on switch. My questions is, on the switch interface that connects to the router interface, do I need to add the "mls trust dscp" command to that interface, so the router does not have to mark the incoming packets on the ethernet interface?
Same question for Data Center design, where the router would be up stream from the access layer switches off our Core 6500 switches.
The trust will preserve the packet QoS marking but it won't impose a marking into the packet. If the packet is coming unmarked from the source, you need to implement some kind of marking with a MQC design of some sort.
In your case, the RTP traffic needs to be marked on ingress at the switch. Either the device itself is sending the packet market and you trust it or you need to implement a policy-map inbound from those devices.
As the packet moves towards the router, it's already marked and all you need is the trust dscp on the switchport facing the router.
The router will leave the packets untouched and markings will be preserved.
At the egress interface on the router, you may want to classify these packets for bandwidth guarantee or priority - you know the drill.