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how switches see tagged frames

Frequent Contributor
Frequent Contributor

Hi all, when using cos for my voice phones, if i have a pc and phone plugged into the same port, the phone is vlan 2, the pc is vlan 1, how does my switch know to put the phone in vlan 2, does it see that the phone has a 802.1p tag, thus meaning its a tagged frame and puts it in vlan 2?

6 Replies 6


It is not the switch, but the IP phone which creates the .1q header and marks the traffic.If you remember the trust boundary is ur ip phone.

as the phone is only setting the 802.1p priority in the frame, is this also creating a .1q tag ?

802.1p is just a standard that talks about how the CoS can be made use of to provide L2QoS.So the IP phone creates a dot1q header and marks the CoS for the voice signal and voice data.

You can set an 802.1p priority without setting an 802.1q tag by leaving the 802.1q part as zero. That is a convention that means the access VLAN, whatever it happens to be. Null tag, if you like.

Cisco phones can go one step further. If they tag the priority as 5 and the dot1q tag as zero, then the switch interprets it as "voice vlan". You configure the port with an access vlan and a different voice VLAN. That allows you to form a sort of 2-vlan implicit "trunk" without having to configure a dot1q tag explicitly in the phone.

Kevin Dorrell


Hi Kevin

thanks for your repsonse, im getting a little closer now, when you say null tag, does that mean there is a tag there but with no info such as vid in there etc? im guessing you mean vlan id of 0 ? the switch will automatically see this tag and put it into the correct vlan ?

That's right, it leaves VID=0

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