User's Manual

438 Features
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, Nortel, 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 a Nortel 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 Nortel 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 encoded in a
Site Specific option. Nortel has provided a list of five possible Site Specific
option codes to implement the Voice Gateway Media Card information.
Only one of the five codes must be configured to carry the information, but
the choice is available to offset the possibility that the option code chosen
has been used for other purposes.
IP acquisition sequence
This section focuses on the mechanics and sequence of the DHCP
message exchange as the IP Phone uses DHCP for IP acquisition.
Although the IP Phone requests many network configuration parameters
as well as an IP address, the following cases focus on the concept of
"how" instead of "what" information is acquired. Also, the IP Phone is used
as the sample client but the situations apply to other DHCP clients as well.
Nortel Communication Server 1000
IP Phones Fundamentals
NN43001-368 05.06 30 April 2010
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