How do you set up priority queueing on a 3400 metro switch?

Unanswered Question
Jan 15th, 2008

running IOS 12.2(40)SE. This seem to do strict priority queueing only which means that that queue will be serviced until it is emptied, then other queues will be serviced.

We will be pumping lots of voice calls through a few of these 3400s on a metro fiber ring and I need to apply the appriopriate QOS for voice. I'm familiar with LLQ in the router world and autoqos in the 3750/3650 switch world, but not with this switch.

Is there a white paper that speaks directly to VoIP on the 3400? The QOS section of the config guide doesn't help when looking for config examples for voice except to state that voice can be mapped to the priority queue, but it is not LLQ based.

I don't want to police voice because the potential of dropped packets could affect all calls, but I need to make sure that voice traffic gets priority over all else - without starving out the other traffic.

I have this problem too.
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Brandon Buffin Wed, 01/16/2008 - 10:25

The 3400 series does not have the concept of LLQ like a router. In order to accomplish something similar, you have to use the "priority" policy-map command along with policing. Take a look at the following link.

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


jkeeffe Wed, 01/16/2008 - 10:34

I understand that - and thank you for responding. However I am extremely leary of policing voice traffic because any dropped packets will affect voice conversation.

Maybe I should set up a policer that is much larger than the total amount of voice traffic I ever expect to have on the link. Say I'll have a possible 100 G-711 calls at any given time. In the LLQ world I'd carve out a priority queue of 10meg to insure all those calls get prioritized. In the 3400 world should I police say at 12meg?

What I'm not clear on is the concept of strict priority queueing - that that queue will be serviced at the expense of all other queues whenever there is traffic in it. Would I expect there always to be voice traffic in the queue if the link is a gig link, and 10mb of voice traffic is going through that queue constantly? Or will the gig interface pull those packets out of the queue so fast that all other non-priority traffic will get transmitted from the other queues just fine?

Brandon Buffin Wed, 01/16/2008 - 10:52

Keep in mind that strict priority queuing is also used on other switches that carry voice such as the 3650/3750. This is accomplished with the "priority-queue" command on these platforms and is generally done without policing. The idea is that bandwidth is plentiful and voice traffic is usually small compared with other types of traffic. I don't know your network and traffic patterns, but I'm guessing you would be safe to put voice in the strict priority queue with no policing.



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