1

Sorry if i sound completely senseless,

In CBWFQ,

q1) Does WRED only make sense if there is more than 1 dscp marking in a queue ?

q2) if traffic_typeA is to be handle with higher priority then traffic_typeB, what would be a better configuration/option below and why >

Option1

-> 1 big queue, with higher drop rate for lower priority traffic

traffic_typeA - tagged AF31

traffic_typeB - tagged AF32

AF31 and AF32 in the same queue

VS

Option 2

-> 2 different queues, with 1 queue having a higher priority than the other.

traffic_typeA - tagged AF31

traffic_typeB - tagged AF41

Both AF31 and AF41 are in different queue

  • RED randomly drops packets in a queue to help prevent TCP global synchronization. – Ron Maupin Mar 25 at 13:45
  • @RonMaupin yeap but if there is only 1 dscp marking / precedence for traffic in the queue, then it does not make sense to have WRED right ? – Noob Mar 25 at 15:35
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    If you have multiple TCP connections using the queue, then RED absolutely makes sense. It prevents tail drop, which is what causes TCP global synchronization of the multiple TCP connections. That has nothing to do with precedence. Cisco has simply enhanced RED with a weight, but even if everything is weighted the same, the benefit of RED still exists. – Ron Maupin Mar 25 at 16:01
  • i see.. thanks! Edited q2) Could you give your advice on q2) ? – Noob Mar 25 at 16:07
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    Your option 1 is handling both traffic types at the same priority, just with different drop probabilities, so it does not seem to fit your second question, which says that one should have a higher priority. – Ron Maupin Mar 25 at 16:40
3

Does WRED only make sense if there is more than 1 dscp marking in a queue ?

Not really. The point of Weighted RED is to assign different weights to different traffic classes. Doesn't make much sense if you only have one class.

What is the difference between 1 big queue, with higher drop rate for lower priority traffic vs. 2 different queues, with 1 queue having a higher priority than the other.

Functionally, nothing. Using one queue makes the hardware design simpler and cheaper.

  • Ron - so it does not make sense for WRED if you only have 1 dscp marking in a queue right ? – Noob Mar 25 at 15:32
  • weights are applied to different classes. If you only have one class, all traffic will have the same weight. – Ron Trunk Mar 25 at 15:34
  • Ron - thanks , i have edited q2) - can you give your opinion on it ? or it still the same functionally ? – Noob Mar 25 at 16:12
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    It doesn't matter how you tag traffic -- it matters how you schedule the different queues/depths. You can configure one queue with different depths or two queues with different priorities to schedule the same. – Ron Trunk Mar 25 at 16:24

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