Tip of the Month

October 2004

VoIP Phones and VLAN

VoIP phones most often are connected to a switched network on their own VLAN. Behavior may differ depending on the manufacturer of the switch and phones in use, but many manufacturers will allow individual ports to be members of more than one VLAN.

When having connectivity issues with VoIP phones one thing to look for is to narrow your initial trace analysis to DHCP packets and ensure proper behavior. Limiting the viewable packets to DHCP may be done by using the ‘select-related’ by protocol or by filtering.

When a phone first boots it will discover a DHCP offered address for the default VLAN of the port the phone is connected to. If the phone is supposed to be connected to a specifically VoIP VLAN then the phone will issue a DHCP release and find the correct VoIP VLAN DHCP server with a discover packet.

If the phone isn’t issuing a DHCP release, there could be negotiation issues between the switch and the phone. Also ensure that the phone receives an IP address for the new VLAN of which it wants to be a member.