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c3750 Priority Queue Egress


I've a question on the PQ egress in a c3750...I read that when you enable PQ on the egress, the scheduler will service the PQ (Q1) until it is empty. Once the PQ has been completely serviced then Q2-4 will be serviced.

The question I have is, if Q1 is a VoIP traffic queue and the call is active for 30mins, then how will Q2-4 get sevice for the past 30mins??


Cisco Employee

Re: c3750 Priority Queue Egress


Your understanding is correct. On 3750, once you put q1 in priority queue, it will be strict priority queue. Scheduler will transmit packets in Q1 until is empty. But that won't cause issue by normal voice traffic, because the bandwidth used for voice traffic is really low. What need to be consider is some PC might send packet marked with EF, which couldstarve other queues.

Lei Tian

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Hall of Fame Super Gold

Re: c3750 Priority Queue Egress

As Lei correctly replied also, you are not considering the bandwidths involved.

Suppose you have 1,000 concurrent VoIp calls at 80 Kbps each, meaning a total of 80 Mbps.

On a Gigabit port, the scheduler will approx. have time to serve 8 times the non-priority queues, for each time it takes to empty the priority queue.

That considered, the best approach is to not configure any priority at all.

Super Bronze

Re: c3750 Priority Queue Egress


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As also confirmed by Lei, Q1 configured to use PQ can starve other queues of all their bandwidth.

One way to help avoid this from happening, is to police (ingress) "PQ" marketed traffic and/or validate "PQ" marked traffic is what's expected.  (I.e. on the latter, TCP traffic with something like DSCP EF or IPPrec 5, might be an invalid marking (assuming DSCP EF and/or IPPrec 5 are directed to PQ).

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