is priority queue in llc always emptied first regardless of congestion

Unanswered Question
Jun 29th, 2009

is priority queue in llc always emptied first regardless of congestion, would the same be true for the hardware priority queue in wrr/srr

I have this problem too.
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Istvan_Rabai Mon, 06/29/2009 - 23:16

Hi Mike,

Collin wants to indicate that the layer3 queuing mechanisms are involved only when the TX-ring is full and the next packet arrives.

This indicates congestion and the LLQ mechanism gets involved.

If there is no congestion (i.e. TX-ring isn't full) then no LLQ action is involved.

On the LLQ:

If there is congestion, of course the priority queue of the LLQ is emptied first to the extent of its configured bandwidth. (The priority queue is policed from above for the bandwidth configured).

Packets exceeding the configured bandwitdth on the priority queue are dropped.



mschooley Tue, 06/30/2009 - 06:58

is this also true for wrr in the hardware queues of a 3560?

Istvan_Rabai Tue, 06/30/2009 - 08:26

Hi Mike,

It is not the same.

In the 3560 there is srr queuing, which means shared round-robin and shaped round-robin, depending on the queuing mode.

In general, both shared round-robin and shaped round-robin queuing limit the bandwidth of the priority queue to prevent bandwidth starvation of non-priority queues.

But in shared round-robin mode, if no packets are in the non-priority queues, then the priority queue can be serviced at up to the line rate.

Also, in shaped round-robin mode, the excess traffic in the priority queue is queued and shaped rather than dropped.

Here's the link to the QoS config guide of the 3560 switches.



mschooley Tue, 06/30/2009 - 09:01

i understand that, but does shared round robin take effect even if the port is not congested, say its a 100mb port and only 30mb used or in proper terminology, the tranmit ring isn't full, does the priority queue still get serviced first.

Istvan_Rabai Tue, 06/30/2009 - 20:54

Hi Mike,

I have a supposition as to how it works, but without a clear Cisco reference I do not want to tell you anything.

I have been looking for a Cisco documentation that tells about how it works, but I still haven't found anything.

I keep on searching....



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