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Network features 435

The IP Phone retrieves the relevant information and uses it to configure

the IP address for the primary TPS and optional secondary TPS.

Either this site-specific option must be present or a similarly encoded

vendor-specific option must be sent. That is, configure the DHCP

server to send one or the other but not both. The choice of using either

vendor-specific or site-specific options enables Windows NT DHCP

servers to be used with the IP Phone. Windows NT servers do not

properly implement the vendor-specific option and as a result, Windows

NT implementations must use the site-specific version.

The format of the option is Type, Length, and Data. The format of the

same as that of the encapsulated vendor-specific option. See “Type (1

octet):” (page 429) .

Operation

DHCP is an extension of BootP. Like BootP, it operates on the

client-server model. However, DHCP has more message types than

BootP. DHCP enables the dynamic allocation of IP addresses to different

clients. It can be used to configure clients by supplying the network

configuration parameters such as gateway or router IP addresses.

In addition, DHCP has a lease system that controls the duration an IP

address is leased to a client. The client can request a specific lease

length, or the administrator can determine the maximum lease length. A

lease can range from one minute to 99 years. When the lease is up or

released by the client, the DHCP server automatically retrieves it and

reassigns it to other clients, if necessary. This is an efficient and accurate

way to configure clients quickly. This saves the administrator from an

otherwise repetitive task. IP addresses can be shared among clients that

do not require permanent IP addresses.

DHCP messages

There are seven different DHCP messages. Each message relays

certain information between the client and server. See Table 109 "DHCP

message types" (page 435).

Table 109

DHCP message types

DHCP Message Types

Description

DHCPDISCOVER Initiates a client request to all servers.

DHCPOFFER Offer from server following client request.

DHCPREQUEST Requests a particular server for services.

DHCPAK Notifies client that requested parameters can

be met.

Nortel Communication Server 1000

IP Phones Fundamentals

NN43001-368 05.06 30 April 2010