Avaya 1110 Fundamentals page 388

Features
Figure 71: IP acquisition sequence: Case 3
Multiple DHCPOFFERS
In some networks, if more than one DHCP server is present, a client can receive multiple
DHCPOFFER messages. Under these situations, the IP acquisition sequence depends on the
client. The client can wait for multiple offers, or accept with the first offer it receives. If it accepts
multiple offers, it compares them before choosing one with the most fitting configuration
parameters. When a decision is made, the message exchange is the same as if there is only
one DHCP server and proceeds as in the previous cases. The servers that were not chosen
to provide the service do not participate in the exchange.
For example, the 2004 IP Phone responds only to DHCPOFFERs that have the same unique
string identifier, "Nortel-i2004-A", as the 2004 IP Phone. This string must appear in the
beginning of the list of Voice Gateway Media Card parameters. Without this string, the 2004
IP Phone does not accept the DHPCOFFER, even if all parameters requested and Voice
Gateway Media Card information are present. If no valid DHCPOFFERs are sent then, the
2004 IP Phone keeps broadcasting in search of a valid offer.
With multiple DHCP servers on the same network, a problem can occur if any two of the servers
have overlapping IP address range and no redundancy. DHCP redundancy is a property of
DHCP servers. This redundancy enables different DHCP servers to serve the same IP address
ranges simultaneously. Administrators must be aware that not all DHCP servers have this
capability.
IP Phone support for DHCP
This section covers the three uses of DHCP (Full, Partial, and VLAN Auto Discovery).
An "2004 IP Phone-aware" DHCP server is needed only for the Full DHCP and VLAN Auto
discovery. An IP Phone can obtain its IP address and subnet mask using Partial DHCP. The
"2004 IP Phone aware" part returns the Node IP and registration port number. In the case of
the DHCP Auto Discovery, it returns the VLAN IDs. Separate DHCP vendor-specific entries
are needed for the Full DHCP data and the VLAN Auto Discovery data. When using the VLAN
Auto Discovery, both Full DHCP and VLAN Auto Discovery must be configured. Full DHCP
388 IP Deskphones Fundamentals
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February 2013