priority queueing default

Unanswered Question
Jul 7th, 2009

Hi, everyone

For priority queueing, we know there are 4 queueing (high, medium,normol,low). And I can configure all unmatched traffic is in default queue.

What I want to know, Can I just configure two queues (high and normal), just one list is point to high queue, other is default queue?

If I just configure one list pointed to high queue, didn't configure the left 3 queues and default queue, what is result?

Very thanks

Tao

I have this problem too.
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Istvan_Rabai Tue, 07/07/2009 - 22:36

Hi Tao,

If you configure 2 access-lists and put their traffic to the high and normal (default) priority queues respectively, then only these 2 queues will be in use.

1. Traffic assigned to the high priority queue will go to the high priority queue.

2. Traffic assigned to the normal priority queue and all undefined traffic will go to the normal priority (default) queue.

3. The other queues will be unused.

If you configure one access-list and assign its traffic to the high priority queue:

1. The traffic defined by this ACL will go to the high priority queue.

2. All remaining traffic will go to the normal priority (default) queue.

3. The other queues will be unused.

You can use the "debug priority" command to check what queue each traffic is directed to.

Cheers:

Istvan

Joseph W. Doherty Wed, 07/08/2009 - 03:16

"For priority queueing, we know there are 4 queueing (high, medium,normol,low). And I can configure all unmatched traffic is in default queue. "

Yes, PQ provides four queues and one queue is used for not explicitly matched traffic. By default, the "normal" queue is used for not explicitly matched traffic, however you can explicitly assign any of the four queues by using the "priority-list default" command.

"What I want to know, Can I just configure two queues (high and normal), just one list is point to high queue, other is default queue? "

You can't really configure the existance of any queues, all four are always available. However, you can match traffic and direct it to just one of the four queues either using the "priority-list interface" or "priority-list protocol" commands, and specify the queue for all other traffic using the "priority-list default" command.

"If I just configure one list pointed to high queue, didn't configure the left 3 queues and default queue, what is result? "

Matched traffic would go into the "high" queue, all other traffic would go into the "normal" queue.

PS:

BTW, since PQ provides absolute priority, you need to realize traffic in a higher (i.e. not just "high") queue can fully "starve" all lower queues from obtaining any bandwidth. If the platform supports it, CBWFQ is often a better choice.

e.g.

policy-map somewhat_like_PQ_but_wont_starve_lower

class high

priority percent 50

class medium

bandwidth remaining percent 81

class normal

bandwidth remaining percent 9

class low

bandwidth remaining percent 1

or

policy-map somewhat_like_PQ_but_wont_starve_lower

class high

bandwidth remaining percent 64

class medium

bandwidth remaining percent 16

class normal

bandwidth remaining percent 4

class low

bandwidth remaining percent 1

NB: there's also an implicit class-default, but above is shown as possible models similar to PQ using CBWFQ, i.e. additional configuration still required.

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