System information

Network features 423

Case 3 is similar to Case 1, except the first two messages have been

eliminated. This reduces the amount of traffic produced on the network.

See Figure 79 "IP acquisition sequence: Case 3" (page 423).

Figure 79

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 IP Phone 2004 responds only to DHCPOFFERs that

have the same unique string identifier, "Nortel-i2004-A", as the IP Phone

2004. This string must appear in the beginning of the list of Voice Gateway

Media Card parameters. Without this string, the IP Phone 2004 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 IP Phone 2004 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

Nortel Communication Server 1000

IP Phones Fundamentals

NN43001-368 05.02 26 May 2009