DHCP options

DHCP defines the message format as an extension to BOOTP for compatibility. DHCP uses the Option field to carry information for dynamic address allocation and to provide additional configuration information to clients.

Figure 269 DHCP option format

Common DHCP options

The following are common DHCP options:

•Option 3—Router option. It specifies the gateway address.

•Option 6—DNS server option. It specifies the DNS server's IP address.

•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 into 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 includes values that correspond to the parameters requested by the client.

•Option 60—Vendor class identifier option. It is used by a DHCP client to identify its vendor, and by a DHCP server to distinguish DHCP clients by vendor class and assign specific IP addresses to the DHCP clients.

•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.

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 and sends it to the server.

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