In this lesson, we’ll take a look at the Voice VLAN and how it works..
Usually, IP phones sit next to a computer on the same desk. They require the same UTP cables as computers and also use Ethernet. If we want to connect them to a switch, we have two options.
You could connect the computer and IP phone using two different cables:
This will work but it has some disadvantages:
- You need to install a new cable from the switchport to the IP phone.
- You will lose a switchport for the IP phone.
To solve this, most IP phones (including Cisco) have a three port switch inside of the IP phone:
- One port connects to the switch.
- One port connects to the computer.
- One (internal) port connects to the phone.
This allows us to connect the IP phone and computer like this:
You probably want to separate the data from the computer and IP phone. This is something we can do with voice VLANs.
The computer will be in a data VLAN, the IP phone will be in the voice VLAN. It will look like this:
Behind the scenes, we have a trunk between our switch and IP phone. The port on the IP phone that connects to the computer is an access port. The IP phone will forward all traffic from the computer to the switch untagged, traffic from the IP phone itself will be tagged. The only two VLANs that are allowed though, are the access and voice VLAN.
If you are familiar with the configuration of VLANs then configuring a voice VLAN is very simple. Let’s configure a switchport where we use VLAN 100 for the computer and VLAN 101 for our IP phone.
First, we have to create the two VLANs:
Now we can configure the interface:
SW1(config)#interface GigabitEthernet 0/1
SW1(config-if)#switchport mode access
SW1(config-if)#switchport access vlan 100
SW1(config-if)#switchport voice vlan 101
We configure the interface in access mode and use VLAN 100 for the computer. The switchport voice vlan command tells the switch to use VLAN 101 as the voice VLAN.
Let’s verify our work. You have to use the show interfaces command for this:
SW1#show interfaces GigabitEthernet 0/1 switchport
Administrative Mode: static access
Operational Mode: static access
Administrative Trunking Encapsulation: negotiate
Operational Trunking Encapsulation: native
Negotiation of Trunking: Off
Access Mode VLAN: 100 (COMPUTER)
Trunking Native Mode VLAN: 1 (default)
Administrative Native VLAN tagging: enabled
Voice VLAN: 101 (VOIP)
Administrative private-vlan host-association: none
Administrative private-vlan mapping: none
Administrative private-vlan trunk native VLAN: none
Administrative private-vlan trunk Native VLAN tagging: enabled
Administrative private-vlan trunk encapsulation: dot1q
Administrative private-vlan trunk normal VLANs: none
Administrative private-vlan trunk associations: none
Administrative private-vlan trunk mappings: none
Operational private-vlan: none
Trunking VLANs Enabled: ALL
Pruning VLANs Enabled: 2-1001
Capture Mode Disabled
Capture VLANs Allowed: ALL
Unknown unicast blocked: disabled
Unknown multicast blocked: disabled
Appliance trust: none
Above you can see that we are using VLAN 100 for the Computers and VLAN 101 for the IP phones.
We can also take a look at the trunk status. Although this will show us that the interface is not-trunking, it does tell us the two VLANs that are used:
SW1#show interfaces GigabitEthernet 0/1 trunk
Port Mode Encapsulation Status Native vlan
Gi0/1 off negotiate not-trunking 1
Port Vlans allowed on trunk
Port Vlans allowed and active in management domain
Port Vlans in spanning tree forwarding state and not pruned
Above we see that VLAN 100 and 101 are allowed on this interface. Although it shows up as non-trunking, keep in mind that in reality, this is a trunk.
In this lesson you have learned how to configure the Voice VLAN and how to verify your work. If you have any questions, feel free to leave a comment in our forum.