Priority queue question

[carl_townshend]

Hi All

I would like some clarification on the priority command on routers etc, people say it has a max bandwidth enforcement, however when I look at the Cisco docs, it reads otherwise, see below quote

To me, this says if there is no other congestion on the link, then the priority traffic would be able to use all the link until something else comes in, can anyone confirm.

cheers

"LLQ with Priority Percent Support Restrictions

Dropping Excess Traffic

If the incoming high priority traffic exceeds the bandwidth percentage calculated by the priority percent command, and there is congestion in the network, the excess traffic is dropped. This is identical to the behavior demonstrated when the priority command uses bandwidth in kbps. In both cases, if the high priority traffic exceeds the bandwidth,"

[Jon Marshall]

If there is no congestion on the link then yes the packets marked with high priority could use excess bandwidth, they just wont be treated as high priority packets 

Jon

[carl_townshend]

Hi Jon

Are you saying they would basically keep the markings etc but going into the FIFO queue and not LLQ so get treated normally ?

[Jon Marshall]

Yes, they would not be remarked, they would simply be treated like all other packets. 

Jon

[Peter Paluch · Accepted solution]

Carl, Jon,

Please allow me to join.

Regarding the implicit policing in the LLQ, we had a wonderful thread with Jon, Joe, and Adam at

https://supportforums.cisco.com/t5/lan-switching-and-routing/llq-bandwidth-provisioning/m-p/1771960/highlight/true#M188944

During the thread, I've made a couple of experiments and summarized them in this PDF:

https://supportforums.cisco.com/t5/lan-switching-and-routing/llq-bandwidth-provisioning/m-p/1771946/thread-id/188930/highlight/true?attachment-id=103213

You might be interested in reading it; it demonstrates what Jon has explained earlier.

Best regards,
Peter

[Joseph W. Doherty]

Carl, the references Peter provides has all the gritty details, but in short, interfaces have an interface FIFO (tx-ring) queue. When this fills, the CBWFQ queues "work".

What this means for LLQ, as long as the tx-ring doesn't overflow into the CBWFQ queues, LLQ's implicit policer doesn't engage, but when it does overflow, the packets pushed into the actual LLQ will be subject to the LLQ implicit policer.