A question about WRED in QoS

[Mitrixsen]

Hello, everyone!

I have a question about how WRED works as I find it very confusing. I understand what WRED is and what goals it aims to achieve during congestion (drop packets, avoid Taildrop and so on).

However, the actual principle of it is quite confusing to me. Consider this graph from NetworkLessons.com

Let's say that this is a queue for the "TCP" class and it has a certain amount of bandwidth reserved and everything necessary for it to work.

Why are we dropping everything once we reach 45 packets in the queue, despite the queue not even being full yet? That doesn't seem very efficient to me. To me, it looks like we just have a bunch of space in the queue that will never be used because of this.

Also, what would this graph mean?

Is this a single queue for two different traffic classes?

Thank you all in advance for your help.

[MHM Cisco World]

These are two different traffic'

One have preference 3 and other 5 

When queue start to full low preference traffic start to discard while the high preference keep forward until one point where it also start to discard.

[Mitrixsen]

Hello!

Thank you for the reply. What queue are we talking about here? I thought that each traffic class has its own queue if we are running CBWFQ.

[MHM Cisco World]

Yes if one queue have multi traffic then WRED weight traffic and select less important to drop.

[Joseph W. Doherty]

The two traffic kinds (in your OP) would be for traffic in the same class distinguished by ToS.  (Using ToS is a WRED option, using either IPPrec or DSCP.)

BTW, a non-LLQ class may have multiple queues if FQ is used.  I believe (?) WRED might be used with FQ.  If so, for WRED purposes, WRED should treat all the traffic in the class as if it were in a single queue.

[Joseph W. Doherty]

Is answer to your first question, how do you know queue has an allocation for more than 45 packets?

Even if it does, you're sure not dropping all packets beyond 45 is "good"?

In answer to your second question, "yes", although perhaps "classes" isn't the best word choice.

BTW, generally I recommend non-QoS experts avoid using (W)RED, as it's difficult to use it well.