VLAN IDs 451When the Automatic VOICE VLAN Discovery using DHCP approach isused, and the IP Phone has been configured as such, the following stepsare automatically taken to obtain the VOICE VLAN ID1. The IP Phones perform an initial DHCP Discovery Request in thedefault VLAN.2. The DHCP server returns a DHCP Ack me ssage with an IP address inthe data 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 configured VLAN ID, the packet passes intothe network and the DHCP server Ack message is passed back.When a DHCP Ack message is received, the IP Phone accepts the offerand saves 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.ATTENTIONVLAN filtering on the telephony port is disabled by default. If tagging is enabledon the telephony port, you can enable VLAN filtering on the telephony port. WhenVLAN filtering is enabled, packets destined for the IP Phone port are filteredbased on the MAC address and the VLAN tag.If VLAN filtering is not enabled on the telephony port, packets destined for the IPPhone port are filtered only on the MAC address. Filtering based on the VLAN tagdoes not occur. This makes the telephony port susceptible to broadcast stormsand a Denial of Service (DOS) attack.Nortel Communication Server 1000IP Phones FundamentalsNN43001-368 01.02 StandardRelease 5.0 20 June 2007Copyright © 2003–2007, Nortel Networks.
