QoS Questions on 3750X stack

flurrball · ‎05-08-2013

Greetings,

I'm trying to review a QoS setup, and I'd like to make sure I fully understand the current setup before I change anything. I'm seeing output drops on two different queue-thresholds, but not sure how packets are making it to one of the queues.

Switch Version

CORE#show ver

Cisco IOS Software, C3750E Software (C3750E-UNIVERSALK9-M), Version 15.0(2)SE, RELEASE SOFTWARE (fc1)

System image file is "flash:/c3750e-universalk9-mz.150-2.SE/c3750e-universalk9-mz.150-2.SE.bin"

cisco WS-C3750X-24 (PowerPC405) processor (revision A0) with 262144K bytes of memory.

Switch Ports Model SW Version SW Image

------ ----- ----- ---------- ----------

* 1 30 WS-C3750X-24 15.0(2)SE C3750E-UNIVERSALK9-M

2 30 WS-C3750X-24 15.0(2)SE C3750E-UNIVERSALK9-M

Model number : WS-C3750X-24T-S

Queue Set

CORE#show mls qos queue-set 1

Queueset: 1

Queue : 1 2 3 4

----------------------------------------------

buffers : 10 10 26 54

threshold1: 138 138 36 20

threshold2: 138 138 77 50

reserved : 92 92 100 67

maximum : 138 400 318 400

Output Drops

This interface I'm checking maps to queue-set 1. Output indicates drops in queue2-threshold1 and queue4-threshold3

CORE#show mls qos int statistics

output queues dropped:

queue: threshold1 threshold2 threshold3

-----------------------------------------------

queue 0: 0 0 0

queue 1: 33051 0 0

queue 2: 0 0 0

queue 3: 0 0 30142

Interface Config

CORE#show run int gi1/0/1

!

interface GigabitEthernet1/0/1

description iSCSI-SW01 Gi1/0/48 (Management)

switchport trunk encapsulation dot1q

switchport trunk allowed vlan 1,70

switchport mode trunk

channel-group 30 mode active

end

CORE#show mls qos interface queueing

GigabitEthernet1/0/1

Egress Priority Queue : disabled

Shaped queue weights (absolute) : 25 0 0 0

Shared queue weights : 25 25 25 25

The port bandwidth limit : 100 (Operational Bandwidth:100.0)

The port is mapped to qset : 1

DCSP/COS maps

CORE#show mls qos maps dscp-output-q

Dscp-outputq-threshold map:

d1 :d2 0 1 2 3 4 5 6 7 8 9

------------------------------------------------------------

0 : 04-03 04-03 04-03 04-03 04-03 04-03 04-03 04-03 04-01 04-02

1 : 04-02 04-02 04-02 04-02 04-02 04-02 03-03 03-03 03-03 03-03

2 : 03-03 03-03 03-03 03-03 02-03 02-03 02-03 02-03 02-03 02-03

3 : 02-03 02-03 03-03 03-03 03-03 03-03 03-03 03-03 03-03 03-03

4 : 01-03 01-03 01-03 01-03 01-03 01-03 01-03 01-03 02-03 02-03

5 : 02-03 02-03 02-03 02-03 02-03 02-03 02-03 02-03 02-03 02-03

6 : 02-03 02-03 02-03 02-03

CORE#show mls qos maps cos-output-q

Cos-outputq-threshold map:

cos: 0 1 2 3 4 5 6 7

------------------------------------

queue-threshold: 4-3 4-2 3-3 2-3 3-3 1-3 2-3 2-3

I can find queue4-threshold3 in the mappings, but how are packets getting mapped to queue2-threshold1? The priority queue is disabled for this interface, so I'm not sure how this queue is dropping packets, according to the maps nothing is mapped to 02-01.

Any light you can shed on this is appreciated.

Thanks,

Mark

Joseph W. Doherty · ‎05-08-2013

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

I can find queue4-threshold3 in the mappings, but how are packets getting mapped to queue2-threshold1? The priority queue is disabled for this interface, so I'm not sure how this queue is dropping packets, according to the maps nothing is mapped to 02-01.

If I remember correctly, Q2 (maps) (stats Q1) is also used for special supervisor packets. BTW, PQ only applies to Q1 (maps) (stats Q0).

Also BTW, if PQ isn't enabled, I would expect its queue to more likely drop its packets, not less.

flurrball · ‎05-08-2013

Thanks for the info Joseph.

So since I probably can't remap the supervisor packets, does it make sense as a next step to increase the thresholds for queue2-threshold1 and queue4-threshold3 to try to reduce the output drops?

Should allow for more buffers in the common pool, and the increased thresholds will give the affected queues a bit more room. Does something like this make sense?

Old Queue-set 1

Queueset: 1

Queue : 1 2 3 4

----------------------------------------------

buffers : 10 10 26 54

threshold1: 138 138 36 20

threshold2: 138 138 77 50

reserved : 92 92 100 67

maximum : 138 400 318 400

New Queue-set 1

Queueset: 1

Queue : 1 2 3 4

----------------------------------------------

buffers : 10 10 26 54

threshold1: 138 160 36 20

threshold2: 138 138 77 50

reserved : 85 85 90 85

maximum : 138 400 318 400

Joseph W. Doherty · ‎05-08-2013

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

Yes, buffer tuning can make sense to mitigate output drops.

If fact, 1st question is do you really need QoS enabled? This is an important question, because often the one shared non-QoS queue, with all the buffer resources, doesn't dropped compared to the the 4 QoS egress queues.

If you do need different traffic treatment, then tuning of buffers may mitigate drops. Even when it cannot reduce the overall drop rate, you can select the traffic that will be dropped first.

As tuning 3750 buffers takes from one queue to give to another, reducing drops in the targeted queue may create or increase drops in another queue or queues.

What you propose might help, although the changes are so minor you might not see much of a difference.