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DHCP options
DHCP uses the same message format as BOOTP, but DHCP uses the Option field to carry information for
dynamic address allocation and to provide additional configuration information to clients.
Figure 284 DHCP option format
Common DHCP options
Common DHCP options:
• Option 3—Router option. It specifies the gateway IP address to be assigned to the client.
• Option 6—DNS server option. It specifies the DNS server IP address to be assigned to the client.
• Option 33—Static route option. It specifies a list of classful static routes (the destination addresses
in these static routes are classful) that a client should add to its routing table. If both Option 33 and
Option 121 exist, Option 33 is ignored.
• Option 51—IP address lease option.
• Option 53—DHCP message type option. It identifies the type of the DHCP message.
• Option 55—Parameter request list option. It is used by a DHCP client to request specified
configuration parameters. The option contains values that correspond to the parameters requested
by the client.
• Option 6o—Vendor class identifier option. A client uses this option to identify the vendor to which
it belongs. With this option, the DHCP server can determine the vendor a client belongs to and
assign an IP address within a specific range.
• Option 66—TFTP server name option. It specifies a TFTP server to be assigned to the client.
• Option 67—Bootfile name option. It specifies the bootfile name to be assigned to the client.
• Option 121—Classless route option. It specifies a list of classless static routes (the destination
addresses in these static routes are classless) that the requesting client should add to its routing table.
If both Option 33 and Option 121 exist, Option 33 is ignored.
• Option 150—TFTP server IP address option. It specifies the TFTP server IP address to be assigned to
the client.
For more information about DHCP options, see RFC 2132 and RFC 3442.
Relay agent option (Option 82)
Some options, such as Option 82, have no unified definitions in RFC 2132.
Option 82 is the relay agent option. It records the location information about the DHCP client. When a
DHCP relay agent or DHCP snooping device receives a client’s request, it adds Option 82 to the request
message before forwarding the message to the server.
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