Multiple priority queues

Unanswered Question
Apr 10th, 2009

Can you have multiple priority queues in the same policy map?

If so, can they total 50% of the available bandwidth?

I have this problem too.
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Giuseppe Larosa Fri, 04/10/2009 - 08:16

Hello Richard,

the answer is yes multiple LLQs can be used.

They will be served in a round-robin fashion and then when all of LLQ are empty the normal queues are served.

With 50% for the LLQ few resources are left for the other queues.

However, CBWFQ is able to use and distribute unused resources.

Hope to help

Giuseppe

wilson_1234_2 Fri, 04/10/2009 - 08:25

Thanks,

The reason I am asking is that our provider only classes "ef" in our Gold CAR package we have for MPLS.

We have Voice and are implementing Video.

I am going to class both as ef and match access-lists.

policy-map QoS

class VOICE

priority percent 15

set dscp ef

class VIDEO

Priority percent 30

set dscp ef

class DATA-Priority

bandwidth 128

set dscp af31

class class-default

set dscp default

fair-queue

random-detect

class-map match-all VIDEO

match access-group 109

class-map match-all VOICE

match access-group 110

class-map match-all DATA

match access-group 111

Will the round robin affect the video or voice calls?

Giuseppe Larosa Fri, 04/10/2009 - 08:32

Hello Richard,

there is a risk of increased jitter on voice packets depending from the intermixing of voice and video packets.

The effects can be greater if video bandwith usage is variable over time.

This may reduce voice quality.

If Video bandwidth usage is quite variable over time you may consider to place it in a non priority queue with enough resources.

Hope to help

Giuseppe

wilson_1234_2 Fri, 04/10/2009 - 10:17

Thanks Joseph.

Giuseppe,

If I have two seperate Priority llq classes and they are matched by access-lists,

How would they affect one antother?

They should both be prioritized, unless this is where the round robin comes in?

if so, then it would be better to change the polict to:

policy-map QoS

class VOICE

priority percent 15

set dscp ef

class VIDEO

bandwidth percent 30

set dscp ef

class DATA-Priority

bandwidth 128

set dscp af31

class class-default

set dscp default

fair-queue

random-detect

class-map match-all VIDEO

match access-group 109

class-map match-all VOICE

match access-group 110

class-map match-all DATA

match access-group 111

Giuseppe Larosa Fri, 04/10/2009 - 11:24

Hello Richard,

>> They should both be prioritized, unless this is where the round robin comes in?

exactly there is not a hierarchy between the two LLQ classes / queues: both should be emptied before processing other queues, but if both have packets waiting to be sent out the interface a form of round robin between the two takes place. (a weighted round robin may be taking in account the respective rates)

This second formulation of the policy-map has only one LLQ queue and you provide resources for the video traffic classes.

In this case voice quality is less affected by the video traffic presence, variability over time.

Hope to help

Giuseppe

Joseph W. Doherty Fri, 04/10/2009 - 12:15

"If I have two seperate Priority llq classes and they are matched by access-lists,

How would they affect one antother?

They should both be prioritized, unless this is where the round robin comes in?"

My understanding, there's no round-robin between LLQ classes. All LLQ traffic is placed into one FIFO queue. If correct, one LLQ class can impact the performance of the other. However, since total LLQ bandwidth allocation is less than available bandwidth, LLQ is dequeued first, and LLQ is policed (per class) if there's congestion, impact likely will be minimal but there may also likely be some. (Actual impact would depend much on the attributes of the data flows, i.e. how bursty, and how the ratio of LLQ offered bandwidth compares to maximum available bandwidth [less to max should perform better].)

In other words, other than the LLQ policers dropping some class traffic, these two policies should provide like performance:

policy-map QoS

class VOICE

priority percent 15

set dscp ef

class VIDEO

priority percent 30

set dscp ef

policy-map QoS

class VOICE_n_VIDEO

priority percent 45

set dscp ef

"if so, then it would be better to change the polict to: "

In your new policy, you've no longer have video in a LLQ class. This will keep video from interleaving with the voice traffic, probably improving voice's performance, but at the same time, degrading video performance.

If the video is streaming video, this might be a fine policy. If the video is something like video conferencing, guaranteed performance might be inadequate.

Joseph W. Doherty Fri, 04/10/2009 - 08:42

"Can you have multiple priority queues in the same policy map? "

LLQ queues, no, LLQ classes, yes.

There's only one physical (FIFO) LLQ queue. Multiple LLQ classes allow you to set different values for the implicit LLQ policers.

"If so, can they total 50% of the available bandwidth?"

Yes.

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