Avaya 1110 Fundamentals page 385

Network features
from the DHCP server by including the Parameter Request List option (Option Code 55) in the
DHCPDISCOVER message.
When the DHCP server sees the broadcast, it responds by broadcasting its own DHCP
message. The server, since it knows more about the network, is able to fill in most of the
information in the message. For example, information such as the server IP address and
gateway IP address are included in their respective fields. Since the client does not have an
IP address yet, the server uses the client's MAC address to uniquely identify it. When the client
sees the broadcast, it matches its MAC address against the one in the message.
DHCP options
DHCP options are the sub-fields of the Options field. They carry additional network
configuration information requested by the client such as the IP address lease length and the
subnet mask.
Each DHCP option has an associated option code and a format for carrying data. Usually the
format is as follows:
Option code Length Data
There are two categories of DHCP options: standard and non-standard. The standard options
are predefined by the industry. The non-standard options are user-defined to fit the needs of
a particular vendor or site.
There are a total of 255 DHCP option codes where option codes 0 and 255 are reserved, 1 to
77 are predefined, 1 to 254 can be used for Vendor Specific Options, and 128 to 254 are
designated for Site Specific Options. This arrangement enables future expansion and is used
as a guideline for choosing option codes.
Vendor Specific/Encapsulated option
The Vendor Specific DHCP options are vendor-defined options for carrying vendor-related
information. It is possible to override predefined standard options; however, doing so can cause
conflict when used with components that follow the industry standard.
A useful option is the standard Vendor Encapsulated option – code 43. It is used to encapsulate
other DHCP options as sub-options. For example, the IP Phone 2004 requires vendor specific
Voice Gateway Media Card information. The vendor, Avaya, decided to carry this information
in one of several Site Specific options and then encapsulate it into option 43. Since the
information is specific to an Avaya product, it is vendor-specific. Once encapsulated, the
information appears as one or more sub-options inside option 43, which the IP Phone
decodes.
Site Specific option
Another way to transport the Voice Gateway Media Card information is through Site Specific
options. These are unused DHCP options that have not been predefined to carry standard
information. Unlike the Vendor Specific options, the information transported is "site" specific
and option codes 128 to 254 are used for encoding.
For IP Phones, the Voice Gateway Media Card information involves the location of the Voice
Gateway Media Card in the network. This varies for different sites and can be implemented in
a Site Specific option. If the Vendor Encapsulation option is used, the information is first
IP Deskphones Fundamentals February 2013 385