Enhanced Vlan Tagging - Nortel 2033 Installation And Operation Manual

Page 488 of 600

Enhanced VLAN Tagging

553-3001-368 Standard 20.00
Appendix B: 802.1Q VLAN description
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 Phones 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 IP Phone reads and saves the VOICE VLAN IDs.
4 The IP Phone rejects the DHCP offer (accepts it but immediately gives
up the lease).
5 The IP Phone 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.
Enhanced VLAN has two main functions:
• Enhance the current Voice VLAN by implementing the hardware VLAN
filter on the IP Phone port (SMP).
• Use TX High Priority Queue (HPQ) and 802.1P VLAN priority to
enhance the traffic control on the IP Phone and PC network interface.
December 2006