Nortel 2001 Installation And Operation Manual page 261

Automatic VOICE VLAN ID configuration
As part of the 802.1Q feature, there is an option to automatically discover the
VOICE VLAN ID using DHCP. This process reduces the configuration steps
since entering data manually (the VOICE VLAN ID) is not required.
When the Automatic VOICE VLAN Discovery using DHCP approach is
used, and the IP Phone has been configured as such, the following steps are
automatically taken to obtain the VOICE VLAN ID:
1 The IP Phone 2001, IP Phone 2002 and IP Phone 2004 perform an initial
DHCP Discovery Request in the default VLAN.
2 The DHCP server returns a DHCP Ack message with an IP address in the
data VLAN and one or more voice VLAN IDs in the vendor-specific
field.
3 The telephone reads and saves the VOICE VLAN IDs.
4 The telephone rejects the DCHP offer (accepts it but immediately gives
up the lease).
5 The telephone reboots and sends a DHCP Discovery Request with the
first VLAN ID from the saved list. This is repeated for each VLAN ID in
the list until a response is received.
This works because the Layer 2 switch discards every DHCP Discovery
Request it receives from the IP Phone if the VLAN ID does not match the
VLAN IDs configured on the port. When the IP Phone sends a DHCP
Discovery Request with the port's configured VLAN ID, the packet passes
into the network and the DHCP server's Ack message is passed back.
When a DHCP Ack message is received, the IP Phone accepts the offer and
saves the IP address and Node IP address.
Appendix A: 802.1Q VLAN description
IP Phones Description, Installation, and Operation
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