Avaya 1110 Fundamentals page 383

Format for IP Phone DHCP site-specific option
This section describes the model-specific, site-specific option for the IP Deskphones. This
option uses the "reserved for site specific use" DHCP options (128 to 254) (refer to RFC 1541
and RFC 1533), and must be returned by the DHCP server as part of each DHCPOFFER and
DHCPACK message for the Internet Telephoneto accept these messages as valid.
The IP Deskphone 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 Deskphone.
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
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 83: DHCP message types
DHCP Message Types
DHCPDISCOVER
DHCPOFFER
DHCPREQUEST
DHCPAK
DHCPNAK
IP Deskphones Fundamentals
Type (1 octet):
Table 83: DHCP message types
Initiates a client request to all servers.
Offer from server following client request.
Requests a particular server for services.
Notifies client that requested parameters can be met.
Notifies client that requested parameters cannot be
met.
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Description
February 2013
Network features
383