QOS - allocation of unused bandwidth in a queue

Unanswered Question
Aug 17th, 2009

In everything I've read about QOS, it is stated that any bandwidth allocated to a particular class which is not used, is available for use by other classes. My question is: How is it distributed amongst the other classes?

For example if I have class matching AF41 which is allocated 2Mbps, and is only using 1Mbps during congestion, is the remaining 1Mbps allocated to that class distributed evenly to all the other classes, or distributed based on some weighting of higher DSCP values (like DSCP based WRED for example)?

I have this problem too.
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Edison Ortiz Mon, 08/17/2009 - 15:56

It is allocated to the remaining traffic and the answer will depend on how the remaining traffic is classified.



patrickjgalligan Mon, 08/17/2009 - 15:59

Hi Edison, thanks for your reply. Can you elaborate? If I have 4 classes:





and AF41 is only using 1Mbps of its 2Mbps allocation, how will the remaining 1Mbps be distributed?



Joseph W. Doherty Mon, 08/17/2009 - 17:38

Assuming your asking about CBWFQ "usused" class bandwidth I believe I've seen conflicting Cisco documentation on this question. Which is, it's either allocated to other demanding classes equally or allocated to other demanding classes proportionally (I believe it's the latter). (NB: actually, I believe, it uses WFQ flow weight rules.)

BTW, with 4 classes with the DSCP AF markings you note, would only directly impact WFQ bandwidth allocations (NB: using IP Prec values), CBWFQ explicit classes bandwidth allocations are independent of ToS settings, beyond possible class matching.

Joseph W. Doherty Tue, 08/18/2009 - 03:10

"Do you have a reference URL?"

Unfortunely, not right at hand. It's the type of information buried, I recall, within whitepapers, technotes and/or troubleshooting documentation.

Mohamed Sobair Tue, 08/18/2009 - 05:39


It depends how much bandwidth you reserved for the:




Assured forwarding AF, Expedite forwarding EF, and the CS class selector , are all used to mark the packet but the question is , what are you going to do with the Marked packet.

for example You set the AF31 (marking) for a certain class, The type of QoS policy (Congestion Managment Mechanism) are you applying would determine the bandwidth distribution/reservation for each class.




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