trunking mode with a Cisco IP Phone

Unanswered Question

Hi !


I'm currently preparing my BCMSN and I have some question about trunking mode.


It is difficult to me to understand the difference between "untagged" mode and none trunking mode.


In my certification guide those mode appear to me as essentially the same.


I'm undertsand in all 4 modes data trafic use native VLAN. When I use the mode where I specify a VLAN ID for voice trafic the voice trafic will travel throw that VLAN, when I use dot1q mode the voice trafic will travel throw VLAN 0, where VLAN 0 (I had on the impression the valid VLAN number was starting from number 1) is not the native VLAN of the switch port. (and it's say "But does'nt required a unique voice VLAN to be created" I'm not sure what that is meaning excatly....) In thoses both mode CoS information will encapsulated in the header of the packet.


When IP phone is not connected to a trunking port, the voice trafic and data trafic will travel in same VLAN. What is the difference with the untagged mode where the voice trafic will travel on same wire, untag with different VLAN information then data trafic on the same switch port. Ans with both untagged and none trunking mode no CoS information appear in the header in voice frames.


Excuse me but I'm a French people and I know sometimes it is my understanding of the written English my base problem.

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aghaznavi Wed, 02/27/2008 - 13:55
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The only difference between the 'dot1p' and 'untagged' modes is the encapsulation of voice traffic. The 'dot1p' mode instructs the switch port to use 802.1P priority tagging for voice traffic and to use the default native VLAN (VLAN 0) to carry all traffic. The 'untagged' mode puts voice packets in the Native VLAN, requiring neither VLAN ID nor a unique voice VLAN, it's like voice VLAN configured same as data VLAN.

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