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mls qos on Catalyst 3750

I can't understand dropping algorithm used by WTD on Catalyst 3750.

Giving the following configuration:

Switch(config)# mls qos queue-set output 2 buffers 40 20 20 20

Switch(config)# mls qos queue-set output 2 threshold 2 40 60 100 200

1. What happens if buffer utilization for queue 2 is between drop-threshold1 and drop-threshold2? Are the packets dropped using a specific algorithm as in WRED?

2. What happens if buffer utilization for queue 2 is between drop-threshold2 and maximum threshold? Are the packets dropped using another algorithm?

3. Is it true that with buffer utilization above maximum threshold all packets are dropped?

Thanks

Stefano

1 ACCEPTED SOLUTION

Accepted Solutions
Purple

Re: mls qos on Catalyst 3750

Stefano,

You wrote: "The problem is that drop-thresolds are percentage values related to the ENTIRE buffer, reserved threshold is related to the specific queue's buffer."

My understanding is that the above is not correct. In fact, I believe it is the opposite: the drop thresholds relate to the specific queue buffer, whereas the reserved threshold relate to the entire buffer memory available. The command reference appears to back this up:

http://www.cisco.com/en/US/products/hw/switches/ps5023/products_command_reference_chapter09186a00805023c6.html#wp2144505

Pls do remember to rate posts.

Paresh

4 REPLIES
Purple

Re: mls qos on Catalyst 3750

Stefano,

My understanding of this is as follows:

- WTD is all about tail-drop, there is no RED mechanism at play here

- to answer your questions, we need to know what DSCP/Cos values you have mapped to which queue and threshold

I'll take an example mapping here in order to explain the behaviour. Let's say you have mapped the follwing:

CoS 3 - queue 2, threshold ID 1

Cos 4 - queue 2, threshold ID 2

CoS 5 - queue 2, threshold ID 3

Given that, the answers to your questions are:

1. If buffer util is between drop-threshold1 and drop-threshold2, all CoS 3 frames will be dropped and all CoS 4 & 5 frames will be accepted.

2. In this case, all CoS 3 and CoS 4 frames will be dropped and all CoS 5 frames will be accepted.

3. Yes, all frames will be dropped.

Pls do remember to rate posts.

Paresh

Community Member

Re: mls qos on Catalyst 3750

Paresh,

thanks a lot for your suggestions!

Anyway I still have some doubt related to WTD thresholds. The sintax is the following:

mls qos queue-set output qset-id threshold queue-id drop-threshold1 drop-threshold2 reserved-threshold maximum-threshold

For drop-threshold1 and drop-threshold2 the range is 1 to 400 percent. For reserved-threshold the range is 1 to 100 percent.

By default, drop-threshold1 and drop-threshold2 for queues 1, 3, and 4 are set to 100

percent. Drop-threshold1 and drop-threshold2 for queue 2 are set to 50 percent. The reserved

thresholds for queues 1, 3, and 4 are set to 50 percent. The reserved

threshold for queue 2 is set to 100 percent. The maximum thresholds for

all queues are set to 400 percent.

With these default settings, queue 2 has drop-threshold1 and drop-threshold2 set to 50 percent and reserved threshold set to 100 percent.

The problem is that drop-thresolds are percentage values related to the ENTIRE buffer, reserved threshold is related to the specific queue's buffer.

So with default settings (1/4 of total buffer for each queue) reserved threshold seems to be HIGHER than drop-thresolds!!!

reserved threshold = 1/4 total buffer

drop thresholds = 50% over 400% total buffer = 1/8 total buffer

Maybe there's something missing...

Thanks

Stefano

Purple

Re: mls qos on Catalyst 3750

Stefano,

You wrote: "The problem is that drop-thresolds are percentage values related to the ENTIRE buffer, reserved threshold is related to the specific queue's buffer."

My understanding is that the above is not correct. In fact, I believe it is the opposite: the drop thresholds relate to the specific queue buffer, whereas the reserved threshold relate to the entire buffer memory available. The command reference appears to back this up:

http://www.cisco.com/en/US/products/hw/switches/ps5023/products_command_reference_chapter09186a00805023c6.html#wp2144505

Pls do remember to rate posts.

Paresh

Community Member

Re: mls qos on Catalyst 3750

Paresh,

I think I get it!

Thanks a lot

Stefano

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