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qos for ip-phone on 3750 switch


I've to enable auto qos for ip phones:

auto qos voip

auto qos voip cisco-phone

auto qos voip cisco-softphone

as per cisco doc, above command automatically enables "mls qos trus cos" and "mls qos trus dscp". with this being said, would switch trust dscp and cos values from PC connected to IP phone as well or it would trust dscp & cos only from ip phone?

what is the best practice to deploy qos on access ports on 3750 or any other switch?

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qos for ip-phone on 3750 switch

btw, I would like the switch to trust dhcp value only from ip phone and soft-phone (installed in PC). is it possible to do this?

qos for ip-phone on 3750 switch

What you are asking about is called a conditional trust where you tell a switch port to only trust specific devices. You can do this by device type:

you can use the command "mls qos trust device cisco-phone" at an interface level to instruct the switch to inspect CDP packets to ensure the device you are trusting is a phone. This is what the "auto qos voip cisco-phone" macro command is doing.

Another way to do a conditional trust is to apply an ingress policy on an interface. You create class-maps and policy-maps globally and then apply them to an interface. The "auto qos voip cisco-softphone" macro command actually creates the policy and applies it to the interface.

You can read up on what the auto qos commands are actually doing for you here:

A later version is here. But the table in the above version is a little clearer.

As far as best practice, well that is a matter of opinion. The Cisco QoS SRND is a very good resource and worth the read. It will lay out several options for you. You need to understand your overall QoS design requirements and objectives before you go applying commands though. You have to think about voice, video, streaming, etc. But you also need to think about missiion critical data traffic that you or your company may care about.

Getting back to the network edge. Personally, I prefer to not trust on the edge. I like to use a class-map which leverages access lists to provide some granular control. More granular than what auto qos voip cisco-softphone gives your, to be sure. Anyway, just my opinion.




HTH -Bill (b) (t) @ucguerrilla

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