Class-map Priority

Unanswered Question
Oct 5th, 2007

Quick question on QoS:

If I have two Class-maps defined that overlap in their filtering, how is the priority handled? For example:

Class Map match-any VoIP

Match protocol rtp audio

Class Map match-any VideoConf

Match protocol rtp payload-type "4"

Match protocol rtp payload-type "34"

Am I correct that these two filters overlap since 'Match protocol rtp audio' is set to match Payload Types 0-23? If so, who would grab traffic that has the PT set to 4?

Thanks

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Kevin Dorrell Fri, 10/05/2007 - 11:23

I think it would depend how the policy was written. The policy that uses these classes would be processed in strict order.

Kevin Dorrell

Luxembourg

mikelsall Fri, 10/05/2007 - 12:56

Thanks Kevin.

In strict order, do you mean in the order that the class-maps are entered under the Policy (as in top to bottom)? The Policy Map just marks the dscp and sets up the queue/bandwidth rules.

i.e.

policy-map Test

class VoIP

priority percent x

set ip dscp ef

class VideoConf

set ip dscp af41

bandwidth remaining percent xx

Based on this config, are you saying that class VoIP would be matched instead of VideoConf if a PT=4 came through because VoIP is listed first?

Mohamed Sobair Fri, 10/05/2007 - 14:19

Hi,

NO, class voip is not matched first because of that , instead class voip is matched first because you are combining LLQ with CB/WFQ at your quing example.

LLQ is always matched first because it does contain priority Statment.

when acongestion occurs, the priority percent of the interface which alreay matching/classified by specific type of traffic are forwarded first out the egress queu till it reaches the maximum priority percentage specified, if still there is an extra priotrized traffic it just get dropped or mapped to the fair-qeue whenever there is no congestion.

After that , the CB/WFQ comes into the rule, the bandwidth statment gurantees a minimum bandwidth that could specific traffic gets whenever a congestion exist.

But always priotrized traffic matched first , which defines (Maximum bandwidth) of your voip traffic.

At the end, The rest of the traffic is then WFQ (Weighted fair queued) , as this treates all traffic fairly based on its usage.

HTH

Mohamed Sobair

royalblues Sat, 10/06/2007 - 01:05

Mohamed,

I think the Policy is executed strictly in the order the classes are defined under the policy i.e if the policy had been written as follows the rtp audio would be matched under the class VideoConf and not under voice

policy-map Test

class VideoConf

set ip dscp af41

bandwidth remaining percent xx

class VoIP

priority percent x

set ip dscp ef

Narayan

Mohamed Sobair Sat, 10/06/2007 - 01:52

Narayan,

How is that? The class-map of VOIP is matching protocol rtp audio.

RTP Audio is not under (video-conf) class?

could you please explain your point further so that I can got it.

Regards,

Mohamed Sobair

royalblues Sat, 10/06/2007 - 04:06

Mohamed,

What i was trying to explain is that it is not necessary that the priority class would always be looked up first but it depends on the order of the class which are defined in the policy map.

If the same match parameters exists in two class maps, it will match the one that is reference first in the policy map

Narayan

Mohamed Sobair Sat, 10/06/2007 - 05:15

Narayan,

Ok, However , this is applicable if the same match parameters exist right? if the Match prameters doesnt exist , still priority class will be forwarded 1st.

Regards,

Mohamed Sobair

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