LLQ Question

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Mar 17th, 2009
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All,


In a policy-map, the keyword "priority" configures LLQ for whatever class it's assigned to. My understanding is that there's a built-in policer for this class, and that policer won't let it go past the bandwidth assignment.


If my policy looked like:


policy-map TEST

class TEST

priority 25


Does the above say "You'll get 25kbps minimum in congestive times, BUT you also can never go faster than 25kbps."? The only time that it will ever go faster is if I apply a burst speed after?


Thanks,

John

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Jon Marshall Tue, 03/17/2009 - 10:02
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John


That is a question of much debate :-).


There was a thread a while back i will try and dig out but suffice to say we came to the conclusion that only when the line is congested is it policed to the allocated bandwidth.


If the line is not congested then it can use more than it's allocated bandwidth.


Jon

John Blakley Tue, 03/17/2009 - 10:04
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We all love theory :-) I'd love to see that thread. I'll do some searching also.


Thanks Jon!


John

John Blakley Tue, 03/17/2009 - 10:08
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Jon,


I think I found it, and guess what....it was a post that I did a few weeks back. :) I'm going to read it all over again.


Thanks!

John

Jon Marshall Tue, 03/17/2009 - 12:10
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Edison


"BTW, priority works all the time, not just during congestion."


What do you mean by this ?


Jon

Edison Ortiz Tue, 03/17/2009 - 12:16
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Jon,


The job of the priority is to de-queue whatever is there before any other queue. The process is done all the time not just during congestion.


__


Edison.

Jon Marshall Tue, 03/17/2009 - 12:18
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Thanks for the clarification.



Joseph W. Doherty Tue, 03/17/2009 - 18:20
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I agree LLQ dequeues before other queues, but this is in reference to software queues. My understanding is, if there's not enough congestion to place packets into the software queues, then there would be nothing for LLQ to dequeue.


From your usage guidelines reference:

"Strict PQ allows delay-sensitive data such as voice to be dequeued and sent before packets in other queues are dequeued."


I suspect this is also why the LLQ policer doesn't rate-limit until there's congestion. If, for example, LLQ was specified at 25%, but if actual LLQ traffic is 50% (also assuming there's no other traffic), and none of this LLQ class traffic is actually placed into LLQ, it's not policed.


Also from your usage guidelines reference:

"When the device is not congested, the priority class traffic is allowed to exceed its allocated bandwidth."

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