RFC 1722 (rfc1722) - Page 2 of 5

RIP Version 2 Protocol Applicability Statement

RFC 1722                  RIP-2 Applicability              November 1994


   length subnet masks, but are not of a size or complexity which
   require the use of the larger, more complex, link-state protocol.

   The remainder of this report describes how each of the extensions to
   RIP-1 may be used to increase the overall usefullness of RIP-2.

3.  Extension Applicability

3.1 Subnet Masks

   The original impetus behind the creation of RIP-2 was the desire to
   include subnet masks in the routing information exchanged by RIP.
   This was needed because subnetting was not defined when RIP was first
   created.  As long as the subnet mask was fixed for a network, and
   well known by all the nodes on that network, a heuristic could be
   used to determine if a route was a subnet route or a host route.
   With the advent of variable length subnetting, CIDR, and
   supernetting, it was no longer possible for a heuristic to reasonably
   distinguish between network, subnet, and host routes.

   By using the 32-bit field immediately following the IP address in a
   RIP routing entry, it became possible to positively identify a
   route's type.  In fact, one could go so far as to say that the
   inclusion of the subnet mask effictively creates a 64-bit address
   which eliminates the network, subnet, host distinction.

   Therefore, the inclusion of subnet masks in RIP-2 allows it to be
   used in an AS which requires precise knowledge of the subnet mask for
   a given route, but does not otherwise require OSPF.

3.2. Next Hop

   The purpose of the Next Hop field is to eliminate packets being
   routed through extra hops in the system.  It is particularly useful
   when RIP is not being run on all of the routers on a network.
   Consider the following example topology:

      -----   -----         -----   -----
      |IR1|   |IR2|         |XR1|   |XR2|
      --+--   --+--         --+--   --+--
        |       |             |       |
      --+-------+-------------+-------+--
        |--------RIP-2--------|

   The Internal Routers (IR1 and IR2) are only running RIP-2.  The
   External Routers (XR1 and XR2) are both running BGP, for example;
   however, only XR1 is running BGP and RIP-2.  Since XR2 is not running
   RIP-2, the IRs will not know of its existance and will never use it



Malkin