Class-default traffic drop in congension

Question

Hi Guys,
Please consider the following example and share your thoughts:
In a MQC deployement, consider that there are following two classes defined:
Class1: EF marking with priority 20%
Class2: AF41 marking with bandwidth 40%
All remaining traffic except above said two classes will go to default class known as class-default.
In case of congestion, which traffic will drop in class-default.
As per my understanding class-default will match all traffic except EF, AF41, CS6 & CS7 (EF is defined in Class1, AF41 is defined in Class2, CS6 & CS7 will be considered as control traffic for 25% reserve bandwidth) and during congestion on all classes (including class-default) class-default will not carry the traffic marked with EF, AF41, CS6 & CS7 instead it will carry the low priority traffic (traffic from class selectors CS0, CS1, CS2, CS3 and CS4 (AF42 + AF43).
Please comment & correct if I am wrong. Do let me know if any other clarity is required on this scenerio.
Thanks.

Answer 1

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Posting
Actually LLQ can exceed its bandwidth allocation, if there's no interface congestion.
Class-default does not directly "consider" other class congestion.  Nor do other classes directly "consider" class-default congestion.
Excluding LLQ, what happens when there's congestion, each class is dequeued proportionally to its class (actually queue) weight relative to other classes except for class-default, pre-HQF, with FQ enabled.  The latter, has flow queues which get dequeued, also proportionally, relative to all the other queues.
Each queue, again excluding LLQ, will drop when the number of packets trying to be enqueued exceed the queue depth (in packets) allocated for that queue.
Specifically for your question, class-2, your AF41 marked traffic, will drop if it exceeds it allocated queue size.  By default, this would be tail drop.  If WRED is enabled, WRED will look at the "average" queue depth when tries to enqueue a new packet and determines, based on its settings, whether to drop that packet.
Does class-default, or other classes, have any affect?  Yes, as whatever share of interface bandwidth being otherwise used will not be available to class-2, and when it's not, class-2 may enqueue when it otherwise would not.