switchport voice vlan : untagged / none / dot1p

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May 20th, 2007
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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!

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vassatrian Mon, 05/28/2007 - 07:42
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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

pankajkulkarni Tue, 05/29/2007 - 02:25
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Hello,


The IEEE 802.1p is an extension of the IEEE 802.1Q (VLANs tagging) standard and they work in tandem. The 802.1Q standard specifies a tag that appends to an Ethernet MAC frame. The VLAN tag has two parts: The VLAN ID (12-bit) and Prioritization (3-bit). The prioritization field was not defined and used in the 802.1Q VLAN standard. The 802.1P defines this prioritization field.


There are 2 possibilities while using dot1p -

1. The IP phone sends voice frames with the VLAN ID set to "0" and the switch sets the appropriate native vlan ID while forwarding it over the LAN.


2. The switch uses CDP to instruct the IP Phone to tag voice traffic with the native vlan ID.


Pankaj

rcast Wed, 05/30/2007 - 15:08
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Thank you guys for your response. Pankaj, what about "untagged" and "none" options. What's the difference between these two?



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