Solved: 3750 ingress queue thresholds

voiper_99 · ‎07-02-2007

Hi, I am trying to understand how the queue thresholds work. Here is an example:

mls qos srr-queue input threshold 1 8 16

mls qos srr-queue input threshold 2 34 66

This is the part that confuses me:

"Each threshold value is a percentage of the total number of queue descriptors allocated for the queue."

What I don't understand is how many queue descriptors are there in the first place?

Thank you.

johnnylingo · ‎07-12-2007

I found the latest 3560 guide, and it explains this much better:

"The queues use WTD to support distinct drop percentages for different traffic classes. Each queue has

three drop thresholds: two configurable (explicit) WTD thresholds and one nonconfigurable (implicit)

threshold preset to the queue-full state. You assign the two explicit WTD threshold percentages for

threshold ID 1 and ID 2 to the ingress queues by using the mls qos srr-queue input threshold queue-id

threshold-percentage1 threshold-percentage2 global configuration command. Each threshold value is a

percentage of the total number of allocated buffers for the queue. The drop threshold for threshold ID 3

is preset to the queue-full state, and you cannot modify it."

So with this example:

mls qos srr-queue input cos-map queue 1 threshold 2 1

mls qos srr-queue input cos-map queue 1 threshold 3 0

mls qos srr-queue input cos-map queue 2 threshold 1 2

mls qos srr-queue input cos-map queue 2 threshold 2 4 6 7

mls qos srr-queue input cos-map queue 2 threshold 3 3 5

mls qos srr-queue input threshold 1 8 16

mls qos srr-queue input threshold 2 34 66

For Q1: CoS value 1 will be dropped when 16% of the buffers are full. CoS value 0 will only be dropped when it is 100% full. The 8 is ignored, since no CoS values were assigned to threshold 1.

For Q2: CoS value 2 will be dropped when 34% of the buffers are full, and values 4 6 7 will be dropped at 66%. CoS values 3 and 5 will only be dropped when it is 100% full.

View solution in original post

jkillion · ‎07-03-2007

Don't think of it as a numeric amount of queue descriptors. What you are specifying here is the percentage of available buffers that are allocated to the queues.

By default, buffers are allocated equally amongst the 4 queues (25% for each). Your cmd above sets Queue set 1, queue 1 to have 8 percent of the bufferes, and queue 2 to have 16 percent.

HTH

Jay Killion, CCIE #17873

voiper_99 · ‎07-03-2007

ahh OK then, thank you for the explanation. However, in my example above (which was taken from a live network), the total percentage of the buffers equates to 124%, so I am just wondering how does that work?

jkillion · ‎07-03-2007

2 different queues. Taken from the following link...

http://www.cisco.com/univercd/cc/td/doc/product/lan/cat3750/12225sec/3750scg/swqos.htm

By default, DSCP values 0-39 and 48-63 are mapped to queue 1 and threshold 1. DSCP values 40-47 are mapped to queue 2 and threshold 1.

By default, CoS values 0-4, 6, and 7 are mapped to queue 1 and threshold 1. CoS value 5 is mapped to queue 2 and threshold 1.

voiper_99 · ‎07-03-2007

"2 different queues."

That's what I thought, but then why would you only allocate 8% and 16% percent for the ifrst queue? What happens to the 76% that is left?

johnnylingo · ‎07-11-2007

I'd love to know the answer to that too. What really confuses is in the 3650 configuration guide, where is says:

"Use the mls qos srr-queue input threshold global configuration command to assign weighted tail-drop

(WTD) threshold percentages to an ingress queue. Use the no form of this command to return to the

default setting."

"If threshold 1 is exceeded, packets with CoS or DSCPs assigned to this threshold are dropped until

the threshold is no longer exceeded. However, packets assigned to threshold 2 continue to be queued and

sent as long as the second threshold is not exceeded."

With the configuration produced by AutoQoS:

mls qos srr-queue input threshold 1 8 16

mls qos srr-queue input threshold 2 34 66

Does this mean then that when 8% of the buffers for Queue 1 packets are full, to drop packets? Or does it not happen until 16%? Likewise is it 34 and 66 for Queue 2?

johnnylingo · ‎07-12-2007