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QoS Questions on 3750X stack

flurrball
Level 1
Level 1

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

3 Replies 3

Joseph W. Doherty
Hall of Fame
Hall of Fame

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.

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

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.

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