Cat 6500 QoS - bandwidth allocation ratios

Unanswered Question
Nov 27th, 2008


If i have a line card that has the queue structure 1P2Q2T and i enable QoS, the default bandwidth allocation ratios Q1:Q2 is 5:255 according to CCO documentation.

Does this means that if the port is 1000Mbps, Q1 will will get a bandwidth of approximately 20Mbps, Q2 approximately 980Mbps? If all of my traffic has dscp=0 and place in Q1, does that mean that the max throughput i can get is 20Mbps, any excess traffic will be dropped even if there is no traffic in Q2?


Eng Wee

I have this problem too.
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Jon Marshall Thu, 11/27/2008 - 08:29


A 1P2Q2T card will use WRR (Weighted Round Robin). WRR allows a queue to use moe than it's allocated bandwidth if the other queues are not using this bandwidth.

Note that the other queuing algorithms ie. SRR, DWRR do not behave in the same way. See this link for more details -


e-chuah Thu, 11/27/2008 - 09:23

Hi Jon,

thanks for the reply. THe URL does answer my question on WRR.

If i have a 1p7q8t that uses DWRR, the default bandwidth allocation is 22:33:45:0:0:0:0, what is the net effect? The description in the URL on DWRR does not really explain what is the net effect like what it did for SRR and WRR?


Eng Wee

Jon Marshall Thu, 11/27/2008 - 09:52


My understanding is that DWRR uses the same algorithm as WRR with the difference being that DWRR keeps track of any deficit from any queues each time through. It then tries to compensate for the deficit the next time through.

However it is a modification to WRR and therefore as far as i know if a queue has unused bandwidth then it can be used by another queue.



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Posted November 27, 2008 at 7:25 AM
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