Follwing commands are entered on int fa0/1 where cisco IP phone is connected and next to IP phone a PC is connected.
(fa0/1)---> Cisco IP Phone --> PC
Enable qos globally
mls qos trust cos
switchport voice vlan 10 dot1p
switchport priority ext cos 3
here we are trusting CoS marking on incoming frames from cisco IP phone as those are marked with dot1p. In next command we are telling IP phone to mark frames coming from PC to be marked as CoS value=3.
But when this frame from PC is received by switch port fa0/1 ( this has CoS vlue of 3 as marked by IP phone ) this CoS will be over-writeen by default CoS setting of that switch port which is 0.
switchport priority extend cos 3 has no effect on the frames sent by PC.
Is my understanding correct?
And if at all we really want these frames to be marked with CoS of 5 just like IP phone what can be done ?
Please share your experience.
Any link on cisco.com?
Thanks in advance
The command switchport priority ext cos 3 will override any CoS setting coming from the PC and will be considered a trusted CoS switchport.
Per your command, any device attached to the phone, will be getting CoS 3 on that switchport.
If you were to use switchport priority ext trust then any CoS setting on the PC or any attached device to the phone will be trusted. If the device is sending CoS 5, that's the CoS setting that will be used and it will not be reset back to CoS 0 - which is the default when the port is untrusted.
Trusted switchports will honor the setting from the end device.
If you have the end device to have CoS just like the IP Phone, then the command:
switchport priority ext cos 5