QOS priority command

avillalva · ‎01-29-2009

Hi all,

I've been handed a config to review and it has something i've not seen before.

what does the priority command do without any paramters??

i.e.

policy-map aaaa

class bbbb

priority

None of the IOS that we have support this command without parameters. Has anyone seen this before and if so what is the behaviour?

Mark Yeates · ‎01-29-2009

Think of the priority command as reserving "a priority queue with a specified amount of available bandwidth for CBWFQ trafficâ€¦To give priority to a traffic class based on the amount of available bandwidth within a traffic policy."

The priority command provides a minimum bandwidth guarantee.

In addition, the priority command implements a maximum bandwidth guarantee.

During congestion conditions, the traffic class is guaranteed bandwidth equal to the specified rate. (Recall that bandwidth guarantees are only an issue when an interface is congested.) In other words, the priority command provides a minimum bandwidth guarantee.

http://www.cisco.com/en/US/tech/tk543/tk757/technologies_tech_note09186a0080103eae.shtml#configuringtheprioritycommand

HTH,

Mark

Nicholas Matthews · ‎01-29-2009

This is a valid command. It essentially gives all of the available bandwidth to that class.

What's that you say? Want to know what happens if you put a bandwidth/priority in another class?

Can't do it.

Spits this message out:

bandwidth kbps/percent command cannot co-exist with strict priority or with priority level in the same policy-map

hth,

nick

avillalva · ‎01-29-2009

Thanks Nick,

Can you point me to any documentation?

I can only find traditional priority command documentation with either the percentage parameter or kbs parameter. I can't find anything that suggests it can be used alone. None of our routers will support this either.

Here is Cisco's command syntax, as you can see the bandwidth-kbps part is not optional.

Syntax Description

bandwidth-kbps

Guaranteed allowed bandwidth, in kbps, for the priority traffic. The amount of guaranteed bandwidth varies according to the interface and platform in use. Beyond the guaranteed bandwidth, the priority traffic will be dropped in the event of congestion to ensure that the nonpriority traffic is not starved.

burst

(Optional) Specifies the burst size, in bytes. The range of the burst is 32 to 2,000,000 bytes.