Switchport voice vlan : untagged / none / dot1p

Question

Hope anyone can shed light on this.
When "switchport voice vlan <keyword>" is configured, a Cisco IP Phone will be instructed via CDP how to deal with voice frames. According to CCNP BCMSN Official Certification Guide, a diagram suggests that when using "untagged" as keyword, IP Phone will use a Special 802.1Q trunk case so that 802.1p bits can be set. It also shows a table which seems to contradict the diagram as the CoS bits column indicates no 802.1p bits are set.
So, can any one tell me the difference in terms of 802.1Q, 802.1p, and native vlan how each mode:untagged/dot1p goes about?
My understanding is that with dot1p IP Phone will use 802.1Q using VLAN 0 (null or "native" vlan) merely for frames to be prioritized using 802.1p bits. As for "untagged" mode, I read somewhere that IP Phone would also use 802.1Q vlanid=1025, again, just for 802.1p bits to be set. But "untagged" seems to me like "none" when it comes to how the IP PHone will forward its voice frames.
Thank you!

Answer 1

My understanding is the following:
with dot1Q - it will trunk using dot1q encapsulation
with dot1p - it will use ISL encapsulation
with untagged - native VLAN configured on the port.
This is how it works:
This is how PC at the back of IP Phone works:
Once native is configured switch will communicate with PC with no vlan tags but will add NATIVE VLAN TAG when packets are forwarded to the LAN. SO when you configure NATIVE VLAN - this is the VLAN that switch add the tag into packets.
At the same time once tagged packets with NATIVE VLAN arrive to switch port destined to PC, switch will remove trunking enxapsulation (otherwise packets will be discarded by PC) and will send packets to PC
Similar rule is applicable to IP Phone.
Regards,
Vakhtang