QoS tuning on 6500 (sup720)

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Apr 30th, 2007
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I'm trying to troubleshoot some performance issues on a 6500 where we have QoS enabled (to support a non-Cisco IP telephony environment).


The following is the interface configuration that uplinks from the core to segment where the IPT equipment is located (pretty much straight from Cisco's QoS SRND).


interface GigabitEthernet4/13

no ip address

speed nonegotiate

wrr-queue bandwidth 5 25 70

wrr-queue queue-limit 10 45 15

wrr-queue random-detect min-threshold 3 50 60 70 80 90 100 100 100

wrr-queue random-detect max-threshold 1 100 100 100 100 100 100 100 100

wrr-queue random-detect max-threshold 2 100 100 100 100 100 100 100 100

wrr-queue random-detect max-threshold 3 60 70 80 90 100 100 100 100

mls qos trust dscp

switchport

switchport trunk encapsulation dot1q

switchport trunk allowed vlan 223

switchport mode trunk


On the port I'm seeing the following output from "show queueing interface..."


Packets dropped on Transmit:


queue dropped [cos-map]

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

1 586883 [0 1 ]

2 0 [2 3 4 ]

3 0 [6 7 ]

4 0 [5 ]


The packets dropped steadily increases in queue 1 over time.


I don't think this is effecting the priority traffic, but it does seem to effect un-prioritiezed traffic (e.g. pings from the monitoring systems at the NOC, and potentially TFTP transfers).


We have tried tweaking the queue-limit parameters to see if we can minimize the drops, but with little difference.


Any suggestions on how to go about tuning the QoS better? Or to get more detail on how to troubleshoot?


Note: port 4/13 is on a 6724 module with a DFC3BXL. QoS is only enabled for Voice (no Video, scavenger-class, etc.. is currently enabled).


Thanks for the help.


- Klaus

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Hieu Cao Mon, 04/30/2007 - 14:43
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Take a look at this document on how-to increase the queue bandwidth. From what you've right now, queue 1 is only processed 5% of the time "wrr-queue bandwidth 5 25 70", 25% for queue 2, and 70% for queue 3. Queue 4 is high priority queue for voice, so it's processed first, until it's emptied, before the switch will service other lower queues.


http://www.cisco.com/en/US/products/hw/switches/ps700/products_tech_note09186a008015bf98.shtml#config_wrr


hth,

hieu



klmueller Mon, 04/30/2007 - 14:53
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Thanks for the link. We have played with the "queue limit" settings some before, but not the "bandwidth." We'll give it a try.


- Klaus

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