c3750 Priority Queue Egress

[choywy]

Hi,

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??

Thanks

[Lei Tian]

Hi,

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.

HTH,
Lei Tian

Sent from Cisco Technical Support iPhone App

[paolo bevilacqua]

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.

[Joseph W. Doherty]

Disclaimer

The  Author of this posting offers the information contained within this  posting without consideration and with the reader's understanding that  there's no implied or expressed suitability or fitness for any purpose.  Information provided is for informational purposes only and should not  be construed as rendering professional advice of any kind. Usage of this  posting's information is solely at reader's own risk.

Liability Disclaimer

In  no event shall Author be liable for any damages whatsoever (including,  without limitation, damages for loss of use, data or profit) arising out  of the use or inability to use the posting's information even if Author  has been advised of the possibility of such damage.

Posting

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).