fair-queue (class-default) tuning

[rlortiz]

I would like some information on fair-queue (class-default) tuning on a core link (6509 running 12.1(13)E9):

class class-default

fair-queue

The router was dropping quite a bit of traffic though the link utilization was not high:

See below:

class-map: class-default (match-any)

536570345 packets, 262435046661 bytes

5 minute offered rate 3724000 bps, drop rate 0 bps

match: any

536570337 packets, 262435044926 bytes

5 minute rate 3724000 bps

queue size 0, queue limit 5127

packets output 535420186, packet drops 1148336

tail/random drops 1148336, no buffer drops 0, other drops 0

fair-queue: per-flow queue limit 1281

The understanding is that the router will allocate maximum of up to dynamic 256 queues (by default).

What would happen if there is more then 256 concurrent flows?

Does it make sense to increase the number of dynamic queues?

Does it make sense to increase a queue size and do the "tail-drop" or configure the "random-detect" option? Which one?

What is the default size of a queue if WFQ is configured for class-default?

Would like to get some help understanding the above output. Especially:

queue size 0, queue limit 5127

fair-queue: per-flow queue limit 1281

Thanks

[gmarogi]

Answers to some of your questions :

What would happen if there is more then 256 concurrent flows?

Multiple flows will be mapped to the same queue

Does it make sense to increase the number of dynamic queues?

If there are more conversations than the number of queues.

Does it make sense to increase a queue size and do the "tail-drop" or configure the "random-detect" option? Which one?

When WFQ is used, the queue size is for the whole WFQ system containing all the queues and not for an individual queue. You cannot use tail-drop with WFQ, you can use WFQ's inherent dropping mechanism or WRED which acts on each queue within the whole WFQ system. Chossing which one depends on the protocols/application that you are using.

What is the default size of a queue if WFQ is configured for class-default?

It is 64 packets for the whole WFQ system which is called as congestive discard threshold.