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Two LLQs in CBWFQ

If you configure two (or more) LLQs in CBWFQ, e.g. one for voice and one for video, how does a router service these queues? Does it:

1. Behave like PQ with one queue having a larger priority than the other, i.e. if it's servicing the second configured queue and packets come into the first queue, it will abandon the second queue and service the first; or

2. Behave like PQ with both queues having the same priority, i.e. if it's servicing the second configured queue and packets come into the first queue, it will finish servicing the second queue before servicing the first.

In relation to the above, have most people had more success with:

1. Voice and video in the same LLQ;

2. Voice and video in two separate LLQs; or

3. Voice in the LLQ and video in one of the CBWFQs.

I know that Cisco has done some testing with bandwidths above 768kbps with CAC and placing voice and video in the same LLQ, not resulting in significant delays added to the voice traffic. I would like to know others' experiences.

Cheers.

8 REPLIES
Bronze

Re: Two LLQs in CBWFQ

You can only configure the "priority" command under one class, so what you are talking about isnt possible.

Recommended configuration in a network with both voice and video is to use the priority command for voice, and bandwidth command for video.

New Member

Re: Two LLQs in CBWFQ

http://www.cisco.com/application/pdf/en/us/guest/netsol/ns17/c649/ccmigration_09186a00800d67ed.pdf

Pages 1-19 to 1-20. It mentions "More than one LLQ can be provisioned....etc". I also remember seeing an example configuration in a Cisco document with two "priority" commands configured in CBWFQ. I thought it was this document but cannot find it. But nevertheless it says it is possible.

Bronze

Re: Two LLQs in CBWFQ

Youre right, it was introduced in 12.2T. I havent seen anyone use it before though.

To answer your question though, since the total bandwidth reserved for both classes must be less than 75% of the total link bandwidth, both queues will be able to be serviced reliably when congested.

However, be aware of the packet sizes of the video transmission, and adhere to the rules of fragmenting those packets on lower speed links.

Re: Two LLQs in CBWFQ

My thoughts behind having multiple priority classes in a policy are so you can classify/match on packets differently and set different actions for each class. Then IOS puts all this into the same priority queue on interface...

I asked this exact same question in DQOS and didn't get a good answer. If I recall right, there is only 1 priority queue per interface so it all goes to same queue in end. Debug priority also shows dropped packets but doesn't mention what class they are from.

New Member

Re: Two LLQs in CBWFQ

Thanks for your feedback on the DQOS class. Thanks also for posting in the WAN section of Netpro. I thought I would put the question up in two sections to get the variety of opinions. Obviously you read up on both sections!

I understand that, if I provision all LLQs to < 75% of link capacity, there should not be any contention due to congestion. I'm also more inclined now to believe that two LLQs would behave in a true PQ-type behaviour, i.e. it will continue serving the current queue until it is empty before moving to the other LLQ, regardless of order of configuration.

I guess if the person teaching DQOS can't answer it, then nobody can.

New Member

Re: Two LLQs in CBWFQ

Folks,

My understanding is that you cannot make 2 PQs with CBWFQ. You can create 2 "priority" entries in the policy-map but I think this just lets additional classes use the same PQ. If you "sh queuing int xxx" you will see that the number of PQs does not change when you make additional "priority" entries in the policy-map. I think it all goes into the same single PQ.

I know you can configure up to 4 PQs but as far as I can tell you cannot do this along side of CBWFQ. If anyone knows anything to the contrary, or knows of any way to actually config 2 PQs (vs 2 priority classes) at the same time as CBWFQ, I'd love to hear about it. I have been trying to do exactly this, but have come to the above conclusion.

New Member

Re: Two LLQs in CBWFQ

The Cisco Press DQoS Book Exam Certification Guide has a section on multiple low latency queues with LLQ.

Regarding queueing is supports jhaluska's contention that you not have more than one PQ. The book says that queueing for multiple low latency queues does not differ from a single low latency queue, and that packets in multiple low latency queues are not re-ordered.

According to the DQoS book, what you do get with multiple low latency queues is multiple policers. You could put voice in one low latency queue and video in another low latency queue. Then, video traffic that exceeds the priority command limit will be policed without policing the voice traffic.

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