CBWFQ limited to 75% of BW

[m.baker]

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

[Georg Pauwen]

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

[m.baker]

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...

[Georg Pauwen · Accepted solution]

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

[m.baker]

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.