I'm in the process of planning a VoIP deployment on my network. The other day, I enabled QoS of one of my 3750 access switches so I could start figuring out what changes needed to be made to support VoIP. Here are the details.
C3750 Software (C3750-I9-M), Version 12.2(20)SE4 is what IOS I'm running, and the only commands for QoS I entered into the switch were:
mls qos map cos-dscp 0 8 16 24 32 46 48 56
Right now, no traffic is marked or classified, but when I enabled QoS on the switch with the "mls qos" command, I noticed that all my network based application take a VERY noticeable nose down in the gutter with regards to performance. I even notice that it takes over a second to save a simple word document to the network. Once QoS is disabled, it behaves as expected.
Based on what I've read, the switch's default settings for QoS shouldn't treat unmarked traffic any differently unless I mark or trust the incoming traffic's CoS or DCSP values.
Can anyone point me in the right direction to troubleshoot this issue? In the end, I want default traffic to not suffer with performance issues, but I want voice traffic to use the âpriority outâ option when a call is made from a VoIP phone.
I think you are running into the default behaviour of a 3750 (plus 2960 & 3560) where the default buffer sizes are not adequate for your network traffic and traffic is being dropped. Use the command 'show platform port-asic stats drop FastEthernet x/x/x' to see if you are seeing egress drops in any of the queues.
This is the default behaviour but it can be tuned, however IOS earlier than 12.2(25)SEE1 has some restrictions on how much buffer space can be allocated. See bug ID CSCsc96037.
I use the following global QoS settings in my 3560/3750 switch template, however you need at least 12.2(25)SEE1:
mls qos queue-set output 1 threshold 1 800 800 50 3200
mls qos queue-set output 1 threshold 2 560 640 100 800
mls qos queue-set output 1 threshold 3 800 800 50 3200
mls qos queue-set output 1 threshold 4 320 800 100 800
There are obviously other settings for DSCP-to-Queue and CoS-to-Queue that should be changed, however these settings change the egress queue thresholds for queueset 1 (the queueset all ports are mapped to by default).