QoS for voice in Catalyst 3750

Unanswered Question
May 4th, 2007

I would like to configure Strict Priority for Voice Traffic LLQ in an interface of a 3750. What I have done in routers, for example to guarantee a BW of 402 Kbps for voice is:

class-map match-all voice-signaling

match access-group 101

class-map match-all voice-traffic

match ip rtp 16384 16383

match ip precedence 5

match access-group 100



policy-map VOICE-POLICY

class voice-traffic

priority 402

class voice-signaling

bandwidth 8

class class-default

However, in 3750 the command priority does not exist inside the Class. HOw I can configure the 3750 to have the same effect?

I have this problem too.
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Brandon Buffin Fri, 05/04/2007 - 05:19

Egress priority queuing is enabled per interface on the 3750 with the "priority-queue out" command. The operation of this command is dependent on other configuration such as mapping COS/DSCP values to egress queues, etc. Take a look at the QoS SRND below. Specifically, see the section Campus QoS Design -> Catalyst 2970/3560/3750 - QoS Considerations and Design.


Hope this helps. If so, please rate the post.


6rlopez_2 Thu, 05/10/2007 - 05:16

Thanks for the link. But I can not find any command to "reserve" bandwidth for voice and signaling in case of congestion. Is it possible?

Brandon Buffin Thu, 05/10/2007 - 05:42

You do not need to reserve a specific bandwidth on the 3750. Congestion does not usually occur on high speed ports such as those on a switch. Congestion normally occurs on slower speed serial links such as WAN links which is why you would need to reserve bandwidth on these links. Priority queuing on the switch simply makes sure that higher priority packets are sent out before lower priority packets.


6rlopez_2 Thu, 05/10/2007 - 05:45

In fact, this is a WAN link (10 M but between 2 different sites) and we have a distributed cluster (Publisher is in one site and Subscriber in the other one). That?s why I want to reserve bandwith to guarantee the Intra-cluster traffic is not going to suffer

Brandon Buffin Thu, 05/10/2007 - 05:49

If you need to reserve bandwidth, I would recommend using a router to terminate the WAN link as opposed to the 3750 switch.


earrigault Thu, 05/10/2007 - 06:08


I'ld be really suprised to not be able to configure a strict priority queue on a 3750 router.

You might already know this but we never know.. there is one sure thing :

You cannot implement a policy containing a strict priority queue on the INPUT of an interface.

Paolo Bevilacqua Sun, 05/13/2007 - 05:52

Actually as bbuffin said, it not surprise at all that 3750 does not supports priority. Unlike the router, it doesn't have a general use memory where to queue and buffer packets. There are per-port queues, but these work in a different way that the router, due to the complications of doing L3 switching in hardware.

So all you have to do (and still is mostly optional) is to configure dscp trust or set, and let auto-qos do the rest. Your voice frames hwill be transmitted before any other non-voice in the (rare) case they arrive at the switch simultaneously.


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