2.2.8 DHCPv4 Option Code 249 (0xF9) - Microsoft Classless Static Route Option

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DHCPv4 clients and DHCPv4 servers that implement this specification use some nonstandard options in their implementation.

The length and the data format for the Microsoft Classless Static Route Option are exactly the same as those specified for the Classless Static Route Option in [RFC3442]; the only difference is that Option Code 249 SHOULD<20> be used instead of or in addition to Option Code 121.

Multiple routes can be sent using the option. Each classless route consists of the Destination descriptor and Router IP address elements. The number of routes included in the option can be determined by processing the option data.

Note that the router IP address is of length 4 bytes, whereas the destination descriptor length is between 1 byte and 5 bytes, depending on the subnet mask. This is described in detail as follows.

This option is sent by the DHCPv4 server to the DHCPv4 client in the DHCPOFFER or the DHCPACK message. It has no effect on subsequent options in that message or in any of the messages sent by the client to the server.


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Option Code

Option Length

CSR-1 Destination descriptor (variable)

...

CSR-1 Router IP address

CSR-N Destination descriptor (variable)

...

CSR-N Router IP address

Option Code (1 byte): This MUST be 249 (0xF9).

Option Length (1 byte): The unsigned length, in bytes, of the option, not including the Option Code and Option Length fields.

CSR-1 Destination descriptor (variable): This is determined in exactly the same way described on the destination descriptor specified on page 4 of [RFC3442].

CSR-1 Router IP address (4 bytes): The IPv4 address of the next-hop router that can be used to reach the destination.

CSR-N Destination descriptor (variable): This is determined in exactly the same way as the destination descriptor specified on page 4 of [RFC3442].

CSR-N Router IP address (4 bytes): The IPv4 address of the next-hop router that can be used to reach the destination.