mls qos on Catalyst 3750

Answered Question
Apr 13th, 2006
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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

Correct Answer by pkhatri about 11 years 4 months ago

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

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pkhatri Thu, 04/13/2006 - 02:11
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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



s-durando Thu, 04/13/2006 - 05:33
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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


Correct Answer
pkhatri Thu, 04/13/2006 - 21:59
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  • Purple, 4500 points or more

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

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