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From QID: SW156

This question is a little confusing to me..

Based upon the config given, how will voice traffic from the phone and data traffic from the PC be handled by the switch??

SW_1#configure terminal
SW_1(config)#mls qos
SW_1(config)#int fa0/1
SW_1(config-if)#mls qos trust cos
SW_1(config-if)#switchport voice vlan 10 dot1p
SW_1(config-if)#switchport priority extend cos 3

The answer provided is: "The switchport will trust the CoS value of the ingress voice and data traffic that comes into the switch port".

As I understand this....

1) "mls qos trust cos" extends the trust boundary to the phone (trust the IP Phone's CoS settings).

2) "switchport voice vlan 10 dot1p"  MAKES NO SENSE. I configured this in a lab and you CANT assign a VLAN # while using "voice vlan dot1p". Im guessing this is a typo. But if they meant dot1p, then Vlan ID is 0.

3) "switchport priority extend cos 3" - basically the switch instructs the phone to use a CoS value of 3 for the PC's traffic?

If those statements are correct, HOW THEN can the switchport trust the CoS of both the voice and data traffic??? Wouldn't using the command in step 3 override all settings??

If item 3 had the command "switchport priority extend trust" , then that would indicate that the PC CoS settings should be trusted, but its not the command they use here, so I am left ripping my hair our (what little I have left).

Can someone please explain this? I would really like to know. This doesnt seem to make any sense.


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