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DHCP options
DHCP extends the message format as an extension to BOOTP for compatibility. DHCP uses the
options field to carry information for dynamic address allocation and provide additional configuration
information for clients.
Figure 18 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. A DHCP client uses this option to identify its vendor.
A DHCP server uses this option to distinguish DHCP clients, and assigns IP addresses to them.
• Option 66—TFTP server name option. It specifies a TFTP server to be assigned to the client.
• Option 67—Boot file name option. It specifies the boot file 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 a client should add into 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.
Custom DHCP options
Some options, such as Option 43, Option 82, and Option 184, have no standard definitions in RFC
2132.
Vendor-specific option (Option 43)
DHCP servers and clients use Option 43 to exchange vendor-specific configuration information.
The DHCP client can obtain the following information through Option 43:
• ACS parameters, including the ACS URL, username, and password.
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