QoS priority

[Mark Bowyer]

I have got a 2800 router and I am using the priority command within my policy-map. I am giving the video conferencing traffic a priority of 30. My question is, does this mean that all of my other traffic can only ever use 70% of the total bandwidth of the link? Is that 30% reserved for video conferencing all of the time, regardless of whether it is being used or not?

[Joseph W. Doherty]

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Posting

No and no, your other traffic can use 100%.

LLQ does NOT actually set aside bandwidth.  I.e. If LLQ isn't using any of its allocated bandwidth, the bandwidth is available to other traffic.

Also, although LLQ sets an implicit policer, the policer only engages when you queue.  I.e. your LLQ traffic can also use up to 100% of the bandwidth.

What LLQ does, if there's congestion, your LLQ packets will be dequeued first, but they will also be policed at the allocated rate.

[paul driver]

Hello Joseph
I was on the understanding the LLQ works even when there is no congestion by giving priority to traffic upto the configured value and only the reserved queues that is non priority queues CAN exceed its configured BW is BW is available

Maybe I am misinterpreting the below cco doc

http://www.cisco.com/en/US/docs/ios/solutions_docs/qos_solutions/QoSVoIP/QoSVoIP.html

Res
Paul

Sent from Cisco Technical Support iPad App

[InayathUlla Sharieff]

Hi Paul,

I agree with Joseph, Usually During congestion conditions, the traffic class is guaranteed bandwidth equal to the specified rate. (Recall that bandwidth guarantees are only an issue when an interface is congested.) In other words, the

priority command provides a minimum bandwidth guarantee.

More info on this:

http://www.cisco.com/en/US/tech/tk543/tk757/technologies_tech_note09186a0080103eae.shtml#configuringtheprioritycommand

HTH

Regards

Inayath

[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

If there's no congestion (i.e. no packets queued) there's nothing for LLQ to prioritize by dequeuing it first.

Unlike explicit policers, the LLQ implicit policer only "triggers" when there's congestion.  (NB: this is documented somewhere, and some time ago, Peter Paluch confirmed with a lab test.  Results are in a post within these forums.)

[paul driver]

Hello Mark,

FYI - I have deleted my original post to you as I dont want to take credit for something which is incorrect, as my interpretation of the LLQ was wrong-

Please mark Joseph and Inayathas posts accordingly.

Although its still a bit ambiguous to me  from reading CCO  docs on this matter after reviewing multiple posts from CSC it does seem in time of congestion the other classes CAN indeed use the LLQ bw allocation if its is not being ultilised

Apologies for the misleading you.

res

Paul

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