Re: QoS and Policy-map.

Answered Question
Jun 29th, 2009
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Hi,

I would like to verify that if there is a policy-map applied outbound in an interface whereby the policy-map looks something like this :-


Policy Map customer-x-out

Class voice

Class video

bandwidth 7000

Class gold

bandwidth 7000

Class silver

bandwidth 30000

Class control

bandwidth 2000

Class class-default

bandwidth 21000


that the service-policy only becomes in effect when there is congestion in the link ? If there is no congestion, then the policy-map does not come to effect. Would this be a correct statemet ?


Pls advice,


InternetB.

Correct Answer by Jon Marshall about 7 years 9 months ago

Correct. The policy map will only take effect when there is congestion on the link.


Jon

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Correct Answer
Jon Marshall Mon, 06/29/2009 - 04:10
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Correct. The policy map will only take effect when there is congestion on the link.


Jon

Joseph W. Doherty Mon, 06/29/2009 - 11:29
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"If there is no congestion, then the policy-map does not come to effect. Would this be a correct statemet ? "


Yes, with regards to basic CBWFQ queuing. Not true if you were also using the CBWFQ policy to mark/police/shape packets (NB: as not done in your policy-map example).


"that the service-policy only becomes in effect when there is congestion in the link ?"


Keeping in mind above answer, logically yes, although even for basic policy queuing (such as your example) actually there's often an interface hardware queue or buffer, that when it overflows, the overflow packets are placed into the CBWFQ queue(s). I.e. some congestion might not derive benefit of your policy queuing. (NB: often interface command to adjust hardware queue/buffer size. Later IOSs are supposed to size default better.)


Also, policy maps can be used to manage queue congestion when the physical link isn't congested, such as for a subinterface or VC and/or under a shaper.

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