Priorty QUE will become effective even without congestion

Unanswered Question
Apr 18th, 2008

As per Cisco literature the CBWFQ comes in to affect only when congestion occurs but I have read some where that priorty queue becomes eefective even there is no congestion.

I have this problem too.
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Brandon Buffin Fri, 04/18/2008 - 07:54

Congestion is required for any type of queuing to take effect, including priority queing. If there is no congestion, packets are sent directly to the hardware queue or transmit ring and out the interface. If the hardware queue is not full, there is no congestion and no need for queuing.

Hope this helps. If so, please rate the post.


lagos Sat, 04/19/2008 - 05:31

This is correct - the priority or LLQ is always used regardless of congestion.

Think of the queue being used in typical fasion to match dscp ef, real-time audio/voice from an IP Phone.

Configure the priority queue for 1kbps. This will result in the match and drop of the interesting UDP packets - even though there is no congestion on the link.

Although there is really only one priority queue per router, in some cases, this mechanism can actually be used to block traffic instead of protect it.



Joseph W. Doherty Thu, 04/24/2008 - 16:40

I recently came across a document on the Cisco site that noted LLQ's policer only becomes active when there's interface congestion.


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