Determining the best QoS burst setting

Answered Question
Mar 26th, 2011

Is there a recommended practice for determining the best QoS burst setting when policing on an (ethernet) interface over a WAN circuit...so you get the best throughput as as close to the cir as possible?

Say you have a 20MB wan circuit and want to police a certain type of traffic to say 5MB we can say this is web traffic. What would be a way to determine the best burst speed for optimal throughput for the policed traffic allowing up to the cir speed.

I have this problem too.
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Correct Answer by boss.silva about 3 years 4 weeks ago

In your case, you are just using the defaults, so that should work fine.

Question is, why are you shaping and policing?

If you just apply the service-policy police to the interface directly, traffic won't exceed the bandwidth anyway.

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Average Rating: 5 (2 ratings)
boss.silva Sun, 03/27/2011 - 06:46

I assume you are trying to determine the burst size for the 5mb policed-rate.

Policing will keep the traffic-rate at the specified value, and will not exceed it. It will be as close to the cir as possible as long as there is traffic to be sent out.

It all depends on the Tc. The default value is 125ms, which means that for a 5mb rate, each 125ms will send 625kbps of traffic. This is good for data traffic (in this case, http) because it is not much affected by serialization.

For networks that have voip implemented, it is recommened to use a Tc of 10ms, to prevent serialization delay because of the line-rate speed compared to the policed burst.

please rate the answer if that satisfies your question.

mbroberson1 Sun, 03/27/2011 - 11:23

Hi Bruno,

Thank You for the reply.

So just to clarify my understanding you are basically saying for traffic such as web traffic just taking the defaults weather I am policing at 2MB, 5MB, or whatever...should work fine?

Here is a sample config I am considering.

class-map match-any police
match access-group name police
class-map match-any parent
match class-map police
!
!
policy-map police
class police
   police cir 5000000
     conform-action transmit
     exceed-action drop
class class-default
  fair-queue
  random-detect
policy-map parent
class class-default
  shape average 20000000
  service-policy police
!
ip access-list extended police
permit ip any any eq 80

int fa0/0.100
bandwidth 20000
ip address x.x.x.x x.x.x.x
service-policy out parent

Regards

Correct Answer
boss.silva Sun, 03/27/2011 - 11:45

In your case, you are just using the defaults, so that should work fine.

Question is, why are you shaping and policing?

If you just apply the service-policy police to the interface directly, traffic won't exceed the bandwidth anyway.

mbroberson1 Sun, 03/27/2011 - 13:09

Hi Bruno,

On an ethernet sub-interface you must shape using a parent policy-map then apply a second policy-map which contains your QOS settings attached to their respected class-maps.

You shape all the traffic to tell the interface how much traffic you are allotted from the provider. Then you use a second policy-map for your QOS settings that's applied to the parent. In my example I will actually be policing a couple of other traffic types and not just the one type in the example.

Here's are a couple of links that somewhat explains it.

http://ardenpackeer.com/qos-voip/qos-applying-cbwfq-to-a-sub-interface/

http://www.cisco.com/en/US/tech/tk543/tk545/technologies_tech_note09186a0080114326.shtml

Regards,

BR

boss.silva Sun, 03/27/2011 - 18:11

Quoting Scott Morris: "Gotta love these things..."


There is always something new to learn in QoS world.


Thanks for the info!

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