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voice vlan and dot1p

Answered Question
Jul 23rd, 2012
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Hi everybody.


I am sorry I am still little confused about voice vlan.


ip phone1-----sw1---trunk-----sw2----ipphone2


1) we configure  switchport voice vlan 2 on both switches sw1 and sw2.


Let say ip phone1 sends a frame to ip phone2.


ip phone adds the dot1q tag  with vlan id  2 and sends it to sw1 which forwards it to sw2.


  Sw2 receives the frame with vlan id 2.


Will sw2 removes the vlan2 tag before forwarding it to ipphone 2 or not?


( My understanding is since all qos functions that need to be performed, have been performed and vlan id tag has served its purpose when frame was received and processed by sw2 as internal switch inside iphone2 does not perform qos so we don't need to send the frame with vlan id tag 2. Iphone2 can look at destination mac and decides the frame is voip frame and processes it. Again this is what I assume. I don't know if this is what really happens)




2) Let say we use

switchport acess vlan4

siwtchport voice dot1p on sw1 and sw2.


Ip phone1  wants to send a frame to iphone2. iphone2 adds the dot1q tag with vlan id=0 and sends it to sw1


sw1 receives the frame replaces the vlan id=0 by data vlan 4.


Sw1 forwards the frame to sw2.


sw2 receives the frame and processes it.


Now what will sw2 do next?  will it replaces the vlan id 4 by vlan 0 and sends it to ip phone2? or will it simply removes the tag carrying vlan id=4 and sends it to iphone2?


Thanks and have a great week.

Correct Answer by Giuseppe Larosa about 5 years 3 weeks ago

Hello Sarah,

1) my understanding is that the frame will be tagged out of SW2 port to ipphone2, the dot1q tag is removed by the mini switch inside the IP phone


2) again my understanding is 802.1p header embedded in an 802.1Q header with vlan-id=0


The mini switch has to manage both traffic destined/originated by the IP phone and traffic to/from the PC connected to the PC port (if present). It is a 3 port mini switch that is driven by CDP  ( or LLDP as an option recently) spoken by the LAN switch port, so keeping these frames distinct has a value.


In more recent IP phones the mini switch of the IP phone can be configured to reject any type of tagged frames received from the PC, or to accept frames with specific vlan tag (this is a form of hardening not only a QoS tool) from the PC port.


Hope to help

Giuseppe

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Correct Answer
Giuseppe Larosa Mon, 07/23/2012 - 13:44
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Hello Sarah,

1) my understanding is that the frame will be tagged out of SW2 port to ipphone2, the dot1q tag is removed by the mini switch inside the IP phone


2) again my understanding is 802.1p header embedded in an 802.1Q header with vlan-id=0


The mini switch has to manage both traffic destined/originated by the IP phone and traffic to/from the PC connected to the PC port (if present). It is a 3 port mini switch that is driven by CDP  ( or LLDP as an option recently) spoken by the LAN switch port, so keeping these frames distinct has a value.


In more recent IP phones the mini switch of the IP phone can be configured to reject any type of tagged frames received from the PC, or to accept frames with specific vlan tag (this is a form of hardening not only a QoS tool) from the PC port.


Hope to help

Giuseppe

sarahr202 Mon, 07/23/2012 - 20:54
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  • Bronze, 100 points or more

Thanks Giueppe.  How are you doing? long time no see.

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