Computer and phone on different vlans but in same access port?

steven smith
steven smith used Ask the Experts™
I'm a little confused on how this works, I'm hoping someone can clarify it for me.  On our network we have VOIP service on VLAN 200, Data on VLAN 300.  Our switchports I believe are defined as access ports, which means they should only have 1 VLAN (I don't know what the VLAN is on the access port - I assume 200 or 300).  But the telephone plugs directly into the switchport then the computer plugs into the PC port on the back of the phone.  How is it that this works?  It seems to my befuddled brain that this means there are 2 seperate VLAN's sending info on the switchport - the phone over 200 the PC over 300.
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It sounds like you are using Cisco switches.  Which will mean that the port can actually be configured with two VLAN ports.  One for Data and one for Voice.  You are correct that the port is configured into Access mode, which is normally set to the Data VLAN.  There is another configuration you can set on the port "switchport voice vlan 123".  This allows the phone to access the voice VLAN and pass data traffic as well.
Steven CarnahanAssistant Vice President\Network Manager
Here is how one of our ports is configured:

interface GigabitEthernet5/2
 description Gi5/2
 switchport access vlan 2
 switchport mode access
 switchport nonegotiate
 switchport voice vlan 100

We have additional configuration for port security and QOS that I left out


Yes, I was able to gain access to the switch and you are correct - it does specify "switchport voice vlan 200".  I take it from your immediately identifying it as a Cisco switch (it is) that this isn't common?  Most layer2/3 switches CAN'T do that?  How would the whole pc-plugged-into-the-phone work in that case - both have to be on the same VLAN or no vlan at all?  Curious about that for new deployments, but trying to figure out the best (and least expensive) way to do a new deployment on current equipement here.  Trying to see if we have a Cisco with 1 free port but 5 computer / phone pairs to connect if we can simply use a standard 10/100/1000 switch plugged into that Cisco port, then the phone / computer pairs plugged into that switch.  The switch isn't VLAN aware, so it should just pass the info along and the port on the Cisco will sort it out and not care about the switch or the fact that there are 10 devices working off it, correct?  Thanks, you have been most helpful so far.
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Steven CarnahanAssistant Vice President\Network Manager
1. The non Cisco (or non-managed Cisco) switch needs to be PoE or you need power bricks for the phones.  
2. If you are going to use another switch for the connections and run them through a single port in the Cisco switch then the port on the Cisco needs to be a trunk (uplink) port
the port on the switch would be trunked and placed on both vlans. The phone we use ( NEC ) needs to be configured with the vlans. the port that the phone plugs into on the switch would be configured with the phone vlan and the port for the computer would be tagged for the second vlan.


Not sure I understand why it wouldn't work - same 2 VLAN ID's (one specified as the voice) - what is different between having 1 pair connected of 5 pairs connected through a switch?
Assistant Vice President\Network Manager
Are you running Cisco phones as well?  

Is the unmanaged switch also a Cisco?

All you should need to do is set the port on the managed switch:

 switchport trunk native vlan 100
 switchport trunk allowed vlan 1,100,200
 switchport mode trunk


Thanks guys, between the lot of you I understand this a LOT better now.

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