QoS

[sqambera]

Hi everyone,

I have a very simple question. I am not too experienced with QoS. We have IP phones in our environment and I want to trust marking of voice packets by IP phones (DSCP) on access switches where IP phones are connected. Based on that same marking I would configure my router to treat voice packets with enough bandwidth. The question I have is about configuring the switch for trusting DSCP marked by IP phones. Would following interface command on switch be enough to do that:

Switch(config-if)# mls qos trust dscp

Also is enabling CDP necessary on the switches to enable marking of voice packets? Is there a command through which we could check on the router that voice packets arriving on it are marked with some DSCP value?

Thanks for taking time to provide answer.

Qamber

[Aseem Anand]

Hi,

Cisco VoIP phones mark traffic at both layer 2 and layer 3. Best practise it to set a conditional trust, what that means is trust CoS only when CDP detects a Cisco VoIP Phone.

mls qos trust device cisco-phone

mls qos trust cos

Switch will trust CoS from the IP phone because of the trust command.

After building the trust you need to configure CoS-To-Dscp map which will convert these CoS values from Phone to dscp. You will configure your trunk links to router/switches to trust dscp thus preserving these settings when passing from one device to another.

By default IP phone tags the signaling traffic with COS value of 3 and RTP traffic with COS value of 5.

You can check cos to dscp mapping on the switch itself:

show mls qos maps cos-dscp
   Cos-dscp map:
        cos:   0  1  2  3  4  5  6  7
     --------------------------------
       dscp:   0  8 16 24 32 40 48 56

Refer to the guide below:

http://www.cisco.com/c/en/us/support/docs/switches/catalyst-3750-series-switches/91862-cat3750-qos-config.html

Aseem

(Please rate if useful)

[sqambera]

Hi Aseem,

Thanks for the helpful post. I still want to know why conditional trust is considered as best practice rather than just enabling the trust for dscp on switch ports connected to phone? I am assuming that once trust for dscp would be enabled the switch will not remark the voice packet with any other dscp value and then we could just configure the router with respective treatment for the packet. Please correct me if I am wrong.

Thanks again for your help.

Qamber