max-reserved-bandwidth - actual effect on traffic

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Aug 1st, 2013
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Hi guys,


Sorry if this is not the right place for QoS questions...


I am wondering if there is actual effect on traffic for this command, or is it simply to warn you how much bandwidth you can reserve.


Let's say that I have a slow link of 1.5mbps. The max-r config is set to default 75%. I have two classes under the policy-map


policy-map qos

class class1

  bandwidth percent 75

class class-default

!

int s0/0

service-policy output qos


Then I have two streams going out of the interface. One falls in class1 category. Another one in class-default. Let say that both streams send traffic as much as they could without any flow control mechnism (e.g. ping x.x.x.x size 1500 timeout 0 repeat 10000000).


My question is, will this actually gurantee 25% for the class-default traffic for the 2nd stream? In your opinion how the traffic will be divided?


Thanks!

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Jose Jara Mon, 08/05/2013 - 02:38
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Hello Difan,


It depends on the IOS version.


the max-reserved-bandwidth is deprecated in HQF, which is the default in IOS 12.4(20T). In HQF a policy can reserve up to 100% of the bandwidth or 99% if class-default is not explicitly defined. According to cisco documentation, PAK priority traffic is protected by HQF, so no need to keep badwidth for them.


In HQF fair-queue schedules all flow equally, previously in CBWFQ was weighted by default with higher precedence getting more bandwidth. Also, in HQF you may specifiy the percentage of the excess BW, for the distribution in the classes. The default is 1, which means that the remaining bandwidth is distributed evenly between the classes.


So, in your case if you are using HQF you should get 1.125 Mbps for class1 and 375 Kbps for the class-default.


Hope this helps,


Jose.

Joseph W. Doherty Mon, 08/05/2013 - 04:11
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In pre-HQF CBWFQ, max-reserved-bandwidth just determine the maximum aggregate of bandwidth you can define for all the non-default classes.


In CBWFQ, bandwidth really set the proportional (de)scheduling weight.


In pre-HQF, if you don't explicity assign bandwidth to class-default, I'm unsure its traffic will receive 1/3 the weight of your class1.

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