Hi every body!
I am sorry to bring up another question. As i am reviewing my book for my exam, some questions came to my mind.
Earlier , i discussed the 'switchport voice vlan dot1p" with net pro Jon, Giuseppe and Lejoe.
The command " switchport voice vlan dot1p" tells ip phone to use vlan 0 for voice and native vlan for pC data.
For voice vlan, vlan id field in dot1q header would be filled with zeros.
But how about if the ip phone has been instructed to trust cos value from pc which is using native vlan. since cos values must be passed , dot1q header is needed in frame from pc. But my question is what would be there in vlan feild? if you say nothing, does nothing mean zeros? if nothing mean zeros then how would switch connected to ip phone distinguish between vlan 0(voice vlan) and native vlan as both are identified with zeros in vlan id field in dot1q header.
Thanks a lot!
A multivlan case occurs when using option
switch voice vlan x
With switchport voice vlan dotlp, a single vlan is used to carry both voice and data, in the above case vlan 3.
My question is if switch send priority -tagged frames as untagged frames to uplink switch, how would the cos values be passed as you said earlier, untagged frames means no dot1q header?
Switch is not sending priority tagged frames to the neigboring switch, it would remark the those priority frames with VID 3, making them vlan tagged frames carrying COS and VID.
If switch is configured to send as untagged frame, how would the cos values be passed?
I dont think it would send untagged frames, if it were sending untagged frames, then no COS information could be passed. COS is not carried beyond the next-hop.However, we must remember Layer 3 precedence is still retained.