RFC 3527 (rfc3527) - Page 2 of 9

Link Selection sub-option for the Relay Agent Information Option for DHCPv4

RFC 3527               Link Selection sub-option              April 2003


   DHCP proxy, it can specify the subnet/link from which to allocate an
   IP address that is different from the IP address with which it
   desires to communicate with the DHCP server.

   Analogous situations exist where the relay agent needs to specify the
   subnet/link on which a DHCP client resides, which is different from
   an IP address that can be used to communicate with the relay agent.
   Consider the following architecture:

      +--------+         +---------------+
      |  DHCP  |     IP x|               |IP y
      | Server |-.......-|  Relay Agent  |----+------------+
      +--------+         |               |    |            |
                         +---------------+    |         +------+
                                              |         |Modem |
                                              |         +------+
                                              |          |    |
                                           +-----+  +-----+ +-----+
                                           |Host1|  |Host2| |Host3|
                                           +-----+  +-----+ +-----+

   In the usual approach, the relay agent would put IP address Y into
   the giaddr of any packets that it forwarded to the DHCP server.
   However, if for any reason, IP address Y is not accessible from the
   DHCP server, this approach will fail.  There are several reasons why
   IP y might be inaccessible from the DHCP server:

      o  There might be some firewall capability in the network element
         in which the relay agent resides that does not allow the DHCP
         server to access the relay agent via IP y.

      o  There might not be an IP y.  An example would be the case where
         there was only one host and this was a point to point link.

   In any of these or other cases, the relay agent needs to be able to
   communicate to the DHCP server the subnet/link from which to allocate
   an IP address.  The IP address, which will communicate to the DHCP
   server the subnet/link information, cannot be used as a way to
   communicate between the DHCP server and the relay agent.

   Since the relay agent can modify the client's DHCP DHCPREQUEST in
   only two ways, the giaddr and the relay-agent-info option, there is a
   need to extend the relay-agent-info option with a new sub-option, the
   link-selection sub-option, to allow separation of the specification
   of the subnet/link from the IP address to use when communicating with
   the relay agent.





Kinnear, et al.             Standards Track