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3COM Auto Voice-VLAN

One of my customers told me today they were not able to make any calls on their Alcatel IP Phones from a remote office location. Their IP Phones were unable to retrieve an IP address from the DHCP server. After several minutes of troubleshooting I’ve pinpointed the problem to their 3Com SuperStack 4500 Switch which just got a software update a few days ago. Comparing the current configuration with the configuration before the update did the trick, the "Voice vlan mac-address" line which is needed to put the IP Phone in the correct VLAN was missing. I’m not an experienced 3Com professional so I had to dig into the technical details to find out why this command is so essential when using VoIP.

There are several ways to configure the correct Voice VLAN for your IP Phones, one of them is to define the VLAN locally on every IP Phone, but I’d prefer to use DHCP options to distribute the correct VLAN information to your IP Phones. 3Com also has a feature to automatically add the correct Voice VLAN when an IP Phone is connected to the switch, this feature is called "Auto Voice-VLAN". How does it actually work and what needs to be configured?

How does it work?

Every Ethernet network device has a MAC-address burned-in at the factory, a 48-bit address space for data-link layer (OSI Layer 2) identification. Every MAC-address is printed in a human-friendly format consisting of 6 groups of 2 hexadecimal digits. The first 12 hexadecimal digits represent the Organizationally Unique Identifier (OUI) which is uniquely assigned to a network device to identify the vendor, for example OUI 00-03-6b identifies a Cisco IP Phone.

3Com’s "Auto Voice-VLAN" feature uses these OUIs to identify IP Phones, it automatically adds or removes the dedicated Voice VLAN from an edge port when the IP phone is connected to a switch port.

What needs to be configured?

1. First of all, define the dedicated Voice VLAN.

[3Com-Switch] vlan 120

2. Enable the Voice VLAN on the switch.

[3Com-Switch] voice vlan 120 enable

3. Define the OUIs for every IP Phone that will be connected to the 3Com switch (Only if they are not already set in the switch’s default configuration).

[3Com-Switch] voice vlan mac-address 0080-9f00-0000 mask ffff-ff00-0000 description Alcatel

4. Enable the Voice feature on every edge port where IP Phones may be connected.

[3Com-Switch] interface Ethernet 1/0/1

[3Com-Switch-Ethernet1/0/1] port-link type hybrid

[3Com-Switch-Ethernet1/0/1] voice vlan enable

That’s all what needs to be done! When an Alcatel IP Phone is connected to Ethernet port 1/0/1, it’s recognized as a Voice enabled device and tagged with Vlan ID 120. Don’t forget to enable Power over Ethernet (PoE) on the interface if needed.

For more information about OUIs and assignments go to the IEEE Standards Association website.

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