QoS on a Catalyst 2950

Well, hacked away using some commands on a 2950 today, and pushed through the QoS exam guide by Wendell Odom for the second time (this time hand-writing notes).  I feel really comfortable now with the theory, and am trying to apply some hands-on training.  Too bad there are not that many QoS labs guides out there.  Here's what I did...I'll try to explain along the way:


2950(config-if)#mls qos trust device cisco-phone

*Feb 28 18:41:56 CST: CDP-PA: version 2 packet sent out on FastEthernet0/1no shut
*Feb 28 18:41:56 CST: %TB-5-TRUST_DEVICE_LOST: cisco-phone no longer detected on port Fa0/1, port set to untrusted. \\This happened when I unplugged the phone\\

2950#sh mls qos int fa0/1
FastEthernet0/1
trust state: not trusted
trust mode: not trusted
COS override: dis
default COS: 0
pass-through: none
trust device: cisco-phone


This is a simple...trust the cos values sent by the ip phone if it is discovered on the port using CDP version 2. The switch here is telling it to overwrite packets sent from any connected pc with a COS of 0, or best effort.


2950(config-if)mls qos trust cos
2950(config-if)mls qos cos 7

FastEthernet0/1
trust state: trust cos
trust mode: trust cos
COS override: dis
default COS: 7
pass-through: none
trust device: none


Here we are trusting the COS markings that come in, and remarking any untagged frames with a COS of 7.


2950(config-if)#mls qos cos override

FastEthernet0/1
trust state: not trusted
trust mode: not trusted
COS override: ena
default COS: 7
pass-through: none
trust device: none


Here we use the override command to say, "I dont care if your tagged with a COS value..we are re-writing you with a COS of 7." You can see the COS override is set to ena...or enabled. This command also turns the trust state to untrusted, so that ALL COS values are overwritten.


2950(config-if)mls qos trust cos

interface FastEthernet0/1
switchport access vlan 19
switchport mode access
mls qos cos 7
mls qos trust cos
spanning-tree portfast
end

2950(config-if)#do sh mls qos int fa0/1
FastEthernet0/1
trust state: trust cos
trust mode: trust cos
COS override: dis
default COS: 7
pass-through: none
trust device: none



This last little bit was just to show that re-using the mls qos trust cos, resets the trust cos, and disables the cos override flag.