Can I configure two Priority Queues for two different traffic classes in the Policy Map?

[pweinhold]

Hi,

In QoS, voice traffic is usually marked EF and placed in a priority queue. But interactive video traffic, like VTC, should also receive priority treatment. Can I put both classes in their own priority queues in the same policy map? I thought there could only be one LLQ, but I'm not sure about it.

An example of the config I'm thinking of is below. Voice would be marked EF, VTC would be marked AF41.

policy-map QOS_QUEUE_EGRESS

class VOICE_EF

  priority percent 20

class VTC_AF41

  priority percent 20

Thanks.

[Joseph W. Doherty]

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Posting

You can, although there still is only one actual LLQ.  What the separate classes allow is bandwidth caps, and other settings, unique to the class.

[pweinhold]

So if there's only one actual LLQ, what does the priority command on the VTC_AF41 class buy me? Is it receiving any kind of "priority" treatment?

[Joseph W. Doherty · Accepted solution]

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.

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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

pweinhold wrote:

So if there's only one actual LLQ, what does the priority command on the VTC_AF41 class buy me? Is it receiving any kind of "priority" treatment?

It it receiving "priority" treatment, yes! That classes traffic is also placed into the single LLQ same as VOICE_EF.

You could have something like:

policy-map QOS_QUEUE_EGRESS

class VOICE_EF_n_VTC_AF41

  priority percent 40

Although overall prioritization and overall bandwidth allowance doesn't change, the actual bandwidth allowance caps are different.  In my example, both classes together are capped at 40% while in your example each class is capped at 20% (although the aggregate allowance is also 40%).  Which is better depends on what restrictions you want to place on the traffic.  In my example, voice might use 5% and video 25%, but in your example the video would be capped at 20%.

[pweinhold]

I see! That makes sense.

So, as a follow-up, how would we "prioritize" amongst other classes of traffic? This occurred to me when I was reading about the Mission-Critical and Best Effort classes (as defined by Cisco's QoS baseline). Presumably you would reserve bandwidth for your Mission-Critical class, and Cisco recommends at 25% reserved bandwidth for the Best Effort class. Neither class would be placed in a priority queue, so a sample config might look something like this:

class Mission-Critical

  bandwidth percent 25

class Best-Effort

  bandwidth percent 25

So, with that config, could you really say that one class was getting any better treatment than the other?

[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

Yes, you could configure them that way and if configured that way, no, neither would obtain better treatment than the other.  If they were the only two classes besides LLQ, they would split available bandwidth 50:50 or 1:1.

If Mission-Critical was 50% and Best-Effort was 10% than they would split bandwidth 50:10 or 5:1.  What's best is really what's necessary to meet your traffic's service requirements.

What we've described is how bandwidth could be proportioned between classes, but for classes that support FQ, you can proportion bandwidth between flow queues rather than class queues.  So, for instance, assuming all the traffic had the same tags,

class class-default

fair-queue

might be used instead of your two classes, Mission-Critical and Best-Effort.

[pweinhold]

Okay, thanks for all the good info.