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Here is the topology that I have:

Phone => (access port) L2 Switch (trunk) => Router

My goal here is to add an EF tag to the packets coming from my phones. Basically, if a packet comes in being in Vlan 30, I want to modify the L3 header and add DSCP EF.

Would that be possible to do that on my L2 switch, or can I only modify the CoS tag ? My switch is a C3750

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More than likely, packets from the phone have the correct DSCP values: EF for the data, and something like AF31 for the control traffic (you really do not want the control traffic marked as EF). Your switch will change the traffic to BE unless you use the mls qos trust dscp interface command.

Yes, you can set DSCP values on the packets with that switch. It can get complex, setting up ACLs, class maps, policy maps, etc., and you can configure VLAN-based QoS (mls qos vlan-based).

Cisco maintains many documents about this. For example:

Cisco Catalyst 3750 QoS Configuration Examples

  • I do not believe the switch would untag the packet if QoS is disabled. It should forward without modifying the packets. In our particular case, we do not have congestion issues on our switches, but our provider requests us to tag packets so they can apply the proper policy. – user53632 Apr 30 at 15:34
  • The new switches will accept the DSCP markings, but your switch will actually set DSCP to BE when QoS is enabled unless you trust DSCP on the interface. I believe that was they way the switches worked until the 3800 series. We have comprehensive QoS, and we have eliminated those switches, but I am familiar with how they worked, and I still have the configurations. Specifically for VoIP, we simply trusted what the phones provided, but we had all the other configurations for various other traffic. – Ron Maupin Apr 30 at 15:39

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