RFC 3527 (rfc3527) - Page 4 of 9

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

RFC 3527               Link Selection sub-option              April 2003


   The sub-option contains a single IP address that is an address
   contained in a subnet. The value for the subnet address is determined
   by taking any IP address on the subnet and ANDing that address with
   the subnet mask (i.e., the network and subnet bits are left alone and
   the remaining (address) bits are set to zero).  This determines a
   single subnet, and when allocating an IP address, all of the other
   related subnets on the same link will also be considered in the same
   way as currently specified for the processing of the giaddr in [RFC
   2131, Section 4.3.1, first group of bullets, bullet 4].

   In scenarios where this sub-option is needed, the relay agent adds it
   whenever it sets the giaddr value (i.e., on all messages relayed to
   the DHCP server).

   When the DHCP server is allocating an address and this sub-option is
   present, then the DHCP server MUST allocate the address on either:

      o  the subnet specified in the link-selection sub-option, or;

      o  a subnet on the same link (also known as a network segment) as
         the subnet specified by the link-selection sub-option.

   The format of the sub-option is:

             SubOpt   Len     subnet IP address
            +------+------+------+------+------+------+
            |  5   |   4  |  a1  |  a2  |  a3  |  a4  |
            +------+------+------+------+------+------+

   A relay agent which uses this sub-option MUST assume that the server
   receiving the sub-option supports the sub-option and uses the
   information available in the sub-option to correctly allocate an IP
   address.  A relay agent which uses this sub-option MUST NOT take
   different actions based on whether this sub-option appears or does
   not appear in the response packet from the server.

   It is important to ensure, using administrative techniques, that any
   relay agent employing this sub-option is directed to only send
   packets to a server that supports this sub-option.

   Support for this sub-option does not require changes to operations or
   features of the DHCP server other than to select the subnet (and
   link) on which to allocate an address.  For example, the handling of
   DHCPDISCOVER for an unknown subnet should continue to operate
   unchanged.






Kinnear, et al.             Standards Track