Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. And see here for current known issues.

New Member

COS – DSCP Help on 3750 Stack

Hi –

I have a 3750 stack that terminates both Avaya phones (PC’s off the phones) and also edge devices like routers and ASA firewalls. I have MLS QOS Enabled globally so the DSCP packet rewrite is also enabled. On the ports that the phones connect to I have configured the access VLAN and the VOICE VLAN and I’ve set the cfg to “mls qos trust cos”. No auto qos, etc just mls qos trust cos on each interface a phones connect to.

From what I read from the guide; that is the correct way if I want to just trust the COS (cos 5 for audio and cos 3 for sig) coming in from the phone and honor it throughout the network. On the trunk links I have also configured, mls qos trust cos. On the links to the edge routers I have configured the ports (Access ports) to mls qos trust dscp.

I also have 4 firewall’s that connect to the switch – I have configured those interfaces to trust the COS and not trust DSCP. On the firewall I have configured the HQF for prioritize the traffic to remote VPN phones. I need to verify if what I’m doing is correct, also should the port going to the ASA Firewall’s be set to trust DSCP or COS? DO I need to set a priority queue on each interface? I’m concerned because recently increased our remote work force and we’re seeing some one-way audio and dropped calls

Ref guide:

Thanks in advance,


VIP Purple

COS – DSCP Help on 3750 Stack

Yes, you have to enable priority queue on 3750 switch interfaces using "

priority-queue out" command.

What is the reason you trust CoS in Firewall or ASA connected switch port ? I would trust DSCP in these switch ports. I would also trust DSCP in every inter-switch trunk links.

VoIP phone connected ports QoS config is OK. I have tested it & refer my notes on that below



New Member

COS – DSCP Help on 3750 Stack

Thanks Rasika,

Awesome link BTW. After reading the 3750 QoS guide it mentioned the same thing about all access ports trusintg dscp.

The firewall ports had already been set to trus cos so I wnated to verify before I changed them. Also the uplinks are in a port channel to a Nexus 7K, I read that the 7K's by default trust the dscp makings so that makes sense.

I will repost if I run into any issues - thanks again for the repsonse.


VIP Purple

COS – DSCP Help on 3750 Stack

No worries Mike, Let us know how it goes

Also please rate the response if it is helpful.