switchport voice vlan : untagged / none / dot1p

Unanswered Question
May 8th, 2007
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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.

Hope anyone can shed a light on this.

Thank you!

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