Page 516 of 630 Appendix B: 802.1Q VLAN description553-3001-368 Standard 26.00 August 2007When the Automatic VOICE VLAN Discovery using DHCP approach isused, and the IP Phone has been configured as such, the following steps areautomatically taken to obtain the VOICE VLAN ID:1 The IP Phones perform an initial DHCP Discovery Request in the defaultVLAN.2 The DHCP server returns a DHCP Ack message with an IP address in thedata VLAN and one or more voice VLAN IDs in the vendor-specificfield.3 The IP Phone reads and saves the VOICE VLAN IDs.4 The IP Phone rejects the DHCP offer (accepts it but immediately givesup the lease).5 The IP Phone reboots and sends a DHCP Discovery Request with thefirst VLAN ID from the saved list. This is repeated for each VLAN ID inthe list until a response is received.This works because the Layer 2 switch discards every DHCP DiscoveryRequest it receives from the IP Phone if the VLAN ID does not match theVLAN IDs configured on the port. When the IP Phone sends a DHCPDiscovery Request with the port’s configured VLAN ID, the packet passesinto 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 andsaves the IP address and Node IP address.Enhanced VLAN TaggingEnhanced VLAN has two main functions:• Enhance the current Voice VLAN by implementing the hardware VLANfilter on the IP Phone port (SMP).• Use TX High Priority Queue (HPQ) and 802.1P VLAN priority toenhance the traffic control on the IP Phone and PC network interface.
