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CBWFQ limited to 75% of BW

I have several classes defined up to a maximum of 75% of bandwidth. My question is "If the remaining 25% is not required by control traffic or any other classes, will one class that needs it be able to use it??? Or will the traffic be limited to a max of 75% for a specific class regardless if the other 25% is used or not ?????

I have a class map that limits traffic to 10% in one class. I expected that the class could go up to 100% if no other class required the remaining bandwidth (including control traffic). But, in reality, if I apply CBWFQ, I never am able to use more then 80% of the bandwidth on the interface !

One more question is "If CBWFQ is only used during congestion,....at which point does it decide that there is congestion????"

Thanks

1 ACCEPTED SOLUTION

Accepted Solutions
VIP Purple

Re: CBWFQ limited to 75% of BW

Hello,

as far as I remember, the Cisco IOS assigns an IP precedence of 6 to routing protocol packets on the control plane. You cannot specify a default queue for routing protocol traffic, it will always be prioritized, even if you specify the max-reserved bandwidth as 100%.

Check out this document:

Understanding How Routing Updates and Layer 2 Control Packets Are Queued on an Interface with a QoS Service Policy

http://www.cisco.com/en/US/tech/tk543/tk544/technologies_tech_note09186a0080094612.shtml

Regards,

GP

4 REPLIES
VIP Purple

Re: CBWFQ limited to 75% of BW

Hello,

indeed when you apply QoS/CBWFQ, you will be using only 75% of the available bandwidth.

To change the available bandwidth use this command in interface configuration mode:

max-reserved-bandwidth xx

QoS/CBWFQ kicks in when an interface experiences congestion, that is when it is presented with more traffic than it can handle. Kind of an abstract description...

Regards,

GP

New Member

Re: CBWFQ limited to 75% of BW

Ok Thanks.,so how are routing updates and control traffic classified as ? default-class ???If that is the case, I may be better off changing the max-reserved -bw to 100% and allocate that 25% to the default class...

VIP Purple

Re: CBWFQ limited to 75% of BW

Hello,

as far as I remember, the Cisco IOS assigns an IP precedence of 6 to routing protocol packets on the control plane. You cannot specify a default queue for routing protocol traffic, it will always be prioritized, even if you specify the max-reserved bandwidth as 100%.

Check out this document:

Understanding How Routing Updates and Layer 2 Control Packets Are Queued on an Interface with a QoS Service Policy

http://www.cisco.com/en/US/tech/tk543/tk544/technologies_tech_note09186a0080094612.shtml

Regards,

GP

New Member

Re: CBWFQ limited to 75% of BW

Thanks ! That's a great document.

My QOS is on a 7500 with RSP and VIPs. I am applying my CBWFQ to ATM sub-interfaces and am running OSPF, IPX RIP/SAP on the wan links. Based on your document, it seems that the routing protocols use the class default...so it should be safe to allocate the remaining 25% to class default in which I setup another service policy that does WFQ...

Thanks for all your help.

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