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bandwidth percent calculations for service policy

I started to put together the service policy shown below.  When I got to the class-default class I tried to add "bandwidth percent 35" but that was rejected by the router and it told me I only had 10 percent left.  Where is that missing 25%?  I thought since all the other traffic added up to 65% I had 35% to work with but it seems that is not the case.  Can someone help me out on how this is calculated?

Thanks,
Diego


policy-map QoS
class voice
  priority percent 5
class 1
  bandwidth percent 20
class 2
  bandwidth percent 20
class 3
  bandwidth percent 20
class class-default
  bandwidth percent 10
!

1 ACCEPTED SOLUTION

Accepted Solutions
Hall of Fame Super Blue

Re: bandwidth percent calculations for service policy

diego@ats-corp.com

I started to put together the service policy shown below.  When I got to the class-default class I tried to add "bandwidth percent 35" but that was rejected by the router and it told me I only had 10 percent left.  Where is that missing 25%?  I thought since all the other traffic added up to 65% I had 35% to work with but it seems that is not the case.  Can someone help me out on how this is calculated?

Thanks,
Diego


policy-map QoS
class voice
  priority percent 5
class 1
  bandwidth percent 20
class 2
  bandwidth percent 20
class 3
  bandwidth percent 20
class class-default
  bandwidth percent 10
!

Diego

You can only allocate 75% of the bandwidth within your policy map because the router reserves the other 25% for control traffic, best effort traffic, routing protocol traffic etc. You can change the reserved amount with the "max-reserved-bandwidth" command -

max-reserved-bandwidth

Jon

5 REPLIES
Hall of Fame Super Blue

Re: bandwidth percent calculations for service policy

diego@ats-corp.com

I started to put together the service policy shown below.  When I got to the class-default class I tried to add "bandwidth percent 35" but that was rejected by the router and it told me I only had 10 percent left.  Where is that missing 25%?  I thought since all the other traffic added up to 65% I had 35% to work with but it seems that is not the case.  Can someone help me out on how this is calculated?

Thanks,
Diego


policy-map QoS
class voice
  priority percent 5
class 1
  bandwidth percent 20
class 2
  bandwidth percent 20
class 3
  bandwidth percent 20
class class-default
  bandwidth percent 10
!

Diego

You can only allocate 75% of the bandwidth within your policy map because the router reserves the other 25% for control traffic, best effort traffic, routing protocol traffic etc. You can change the reserved amount with the "max-reserved-bandwidth" command -

max-reserved-bandwidth

Jon

New Member

Re: bandwidth percent calculations for service policy

Thanks Jon, that was very helpful.  One more question.  The article you sent me to says that the extra bandwidth is used for, among other types of trafffic, for best effort.  I thought that best effort traffic would fall under class-default.  What is the difference between best effort and class-default traffic? Thanks, Diego

Hall of Fame Super Blue

Re: bandwidth percent calculations for service policy

diego@ats-corp.com

Thanks Jon, that was very helpful.  One more question.  The article you sent me to says that the extra bandwidth is used for, among other types of trafffic, for best effort.  I thought that best effort traffic would fall under class-default.  What is the difference between best effort and class-default traffic? Thanks, Diego

Diego

Really good question. Best effort will fall under class-default. But what would happen if you didn't configure class-default in your policy map. Best effort traffic still needs bandwidth and basically that's why Cisco reserved 25% of the available bandwidth ie. so if you configured your policy map without a class default then best effort traffic would still get 25% which was the Cisco recommendation.

Jon

Hall of Fame Super Blue

Re: bandwidth percent calculations for service policy

Diego

As a follow up. From Cisco configuration doc -

CBWFQ Bandwidth Allocation

The sum of all bandwidth allocation on an interface cannot exceed 75 percent of the total available interface bandwidth. The remaining 25 percent is used for other overhead, including Layer 2 overhead, routing traffic, and best-effort traffic. Bandwidth for the CBWFQ class-default class, for instance, is taken from the remaining 25 percent. However, under aggressive circumstances in which you want to configure more than 75 percent of the interface bandwidth to classes, you can override the 75 percent maximum sum allocated to all classes or flows using the max-reserved-bandwidth command. If you want to override the default 75 percent, exercise caution and ensure that you allow enough remaining bandwidth to support best-effort and control traffic, and Layer 2 overhead.

Note that the class-default class is taken from the remaining 25% per-cent.

Jon

New Member

Re: bandwidth percent calculations for service policy

So it seems that it is best not to define the bandwidth for best effort in the class map.  I do want to be aggressive and take a little more for my classes.  Therefore what I will try is to set max-reserved bw to 85 and remove the class-default bandwidth line from the policy-map.  This way I will have an extra 10% for my classes and still keep 15% for that other stuff.  Worse that can happen is that I have to roll back the config!!  A little experimenting never hurt...

Thanks,

Diego

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