12-17-2003 04:33 AM - edited 03-02-2019 12:24 PM
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
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12-17-2003 05:46 AM
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
12-17-2003 05:20 AM
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
12-17-2003 05:35 AM
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...
12-17-2003 05:46 AM
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
12-17-2003 06:11 AM
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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