Aux and Native VLANs for voice 802.1q

Answered Question
Jul 20th, 2007

Hi all,


So I have the below (also in diagram if you like)


switch---------ipphone---------pc


I have the following config


set vlan 20 3/1

set port auxiliaryvlan 3/1 10


When a packet hits the phone from the PC does it get an 802.1q header assigned to it before it goes over the trunk to the switchport?, or does it go over the trunk as just a standard ethernet frame?


Also, if it is untagged, I can't see the need for the two following commands :-


set port qos 3/1 trust-ext untrusted

set port qos 3/1 cos-ext 0


?


Many thx indeed,

Ken



Correct Answer by AJAZ NAWAZ about 9 years 7 months ago

set port qos 3/1 trust-ext untrusted

### instructs phones to not trust cos coming from PC ###


set port qos 3/1 cos-ext 0

###resets cos to specified value, in this case 0###


As for trunking to an Cisco IP phone - afaik it is not possible. I cannot say for non Cisco phone since I only have played with 79xx series.

Correct Answer by AJAZ NAWAZ about 9 years 7 months ago

Dear Ken,


Ethernet frames sent from the PC to the phone are untagged. These untagged frames are then forwarded to the network switch.


Ethernet frames sent from the IP phone to the directly connected switch port, are sent 802.1q tagged (This is known as the Voice or Auxillary VLAN).


The network switch (Not the switch built into the phone), sends traffic destined for the phone as tagged. The network switch sends frames destined for the PC (which is attached to the phone), as untagged. Since there is just one set of tagged and untagged frames - there is no 'Trunk' as such. Try issue the show trunk command on a port directly connected to an IP Phone. A trunk would be necessary if there was an additional VLAN. The purpose for this is to reduce the 'Trunking' overhead as it was not necessary. It does make sense - efficient engineering.


With regards to the other commands stated within the post (trust qos), they are simply to do with setting the trust boundary for CoS which is of course used in prioritizing Voice traffic as it is sensitive to delay, so we need this. These command sets allow us to use or disregard the CoS value set by devices which are sending the switch frames for forwarding e.g IP phone.


If you require any further information or links please let us know.


hth

Ajaz

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Correct Answer
AJAZ NAWAZ Fri, 07/20/2007 - 03:53

Dear Ken,


Ethernet frames sent from the PC to the phone are untagged. These untagged frames are then forwarded to the network switch.


Ethernet frames sent from the IP phone to the directly connected switch port, are sent 802.1q tagged (This is known as the Voice or Auxillary VLAN).


The network switch (Not the switch built into the phone), sends traffic destined for the phone as tagged. The network switch sends frames destined for the PC (which is attached to the phone), as untagged. Since there is just one set of tagged and untagged frames - there is no 'Trunk' as such. Try issue the show trunk command on a port directly connected to an IP Phone. A trunk would be necessary if there was an additional VLAN. The purpose for this is to reduce the 'Trunking' overhead as it was not necessary. It does make sense - efficient engineering.


With regards to the other commands stated within the post (trust qos), they are simply to do with setting the trust boundary for CoS which is of course used in prioritizing Voice traffic as it is sensitive to delay, so we need this. These command sets allow us to use or disregard the CoS value set by devices which are sending the switch frames for forwarding e.g IP phone.


If you require any further information or links please let us know.


hth

Ajaz

kfarrington Fri, 07/20/2007 - 04:03

Hi Ajaz,


That is a fantastic reply. Thankyou.



So if the data from the PC, is untagged, the following commands for the "trust-ext" and "cos-ext" are irellevant when you use the aux vlan and would only really be needed if you used a non-cisco/cisco phone and configured a real 802.1q trunk?? would that be the correct assumption ????


set port qos 3/1 trust-ext untrusted

set port qos 3/1 cos-ext 0





Once again, Many thx indeed for the reply. I think im slowly getting there :)


Regards,

Ken

Correct Answer
AJAZ NAWAZ Fri, 07/20/2007 - 10:59

set port qos 3/1 trust-ext untrusted

### instructs phones to not trust cos coming from PC ###


set port qos 3/1 cos-ext 0

###resets cos to specified value, in this case 0###


As for trunking to an Cisco IP phone - afaik it is not possible. I cannot say for non Cisco phone since I only have played with 79xx series.

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