rfc1723.txt

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   The immediate next hop IP address to which packets to the destination
   specified by this route entry should be forwarded.  Specifying a
   value of 0.0.0.0 in this field indicates that routing should be via
   the originator of the RIP advertisement.  An address specified as a
   next hop must, per force, be directly reachable on the logical subnet
   over which the advertisement is made.

   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.  A
   simple example is given in Appendix A.  Note that Next Hop is an
   "advisory" field.  That is, if the provided information is ignored, a
   possibly sub-optimal, but absolutely valid, route may be taken.  If
   the received Next Hop is not directly reachable, it should be treated
   as 0.0.0.0.

3.5 Multicasting

   In order to reduce unnecessary load on those hosts which are not
   listening to RIP-2 messages, an IP multicast address will be used for
   periodic broadcasts.  The IP multicast address is 224.0.0.9.  Note
   that IGMP is not needed since these are inter-router messages which
   are not forwarded.




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RFC 1723                     RIP Version 2                 November 1994


   In order to maintain backwards compatibility, the use of the
   multicast address will be configurable, as described in section 4.1.
   If multicasting is used, it should be used on all interfaces which
   support it.

3.6 Queries

   If a RIP-2 router receives a RIP-1 Request, it should respond with a
   RIP-1 Response.  If the router is configured to send only RIP-2
   messages, it should not respond to a RIP-1 Request.

4. Compatibility

   RFC 1058 showed considerable forethought in its specification of the
   handling of version numbers.  It specifies that RIP messages of
   version 0 are to be discarded, that RIP messages of version 1 are to
   be discarded if any Must Be Zero (MBZ) field is non-zero, and that
   RIP messages of any version greater than 1 should not be discarded
   simply because an MBZ field contains a value other than zero.  This
   means that the new version of RIP is totally backwards compatible
   with existing RIP implementations which adhere to this part of the
   specification.

4.1 Compatibility Switch

   A compatibility switch is necessary for two reasons.  First, there
   are implementations of RIP-1 in the field which do not follow RFC
   1058 as described above.  Second, the use of multicasting would
   prevent RIP-1 systems from receiving RIP-2 updates (which may be a
   desired feature in some cases).  This switch should be configurable
   on a per-interface basis.

   The switch has four settings: RIP-1, in which only RIP-1 messages are
   sent; RIP-1 compatibility, in which RIP-2 messages are broadcast;
   RIP-2, in which RIP-2 messages are multicast; and "none", which
   disables the sending of RIP messages.  The recommended default for
   this switch is RIP-1 compatibility.

   For completeness, routers should also implement a receive control
   switch which would determine whether to accept, RIP-1 only, RIP-2
   only, both, or none.  It should also be configurable on a per-
   interface basis.

4.2 Authentication

   The following algorithm should be used to authenticate a RIP message.
   If the router is not configured to authenticate RIP-2 messages, then
   RIP-1 and unauthenticated RIP-2 messages will be accepted;



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RFC 1723                     RIP Version 2                 November 1994


   authenticated RIP-2 messages shall be discarded.  If the router is
   configured to authenticate RIP-2 messages, then RIP-1 messages and
   RIP-2 messages which pass authentication testing shall be accepted;
   unauthenticated and failed authentication RIP-2 messages shall be
   discarded.  For maximum security, RIP-1 messages should be ignored
   when authentication is in use (see section 4.1).

   Since an authentication entry is marked with an Address Family
   Identifier of 0xFFFF, a RIP-1 system would ignore this entry since it
   would belong to an address family other than IP.  It should be noted,
   therefore, that use of authentication will not prevent RIP-1 systems
   from seeing RIP-2 messages.  If desired, this may be done using
   multicasting, as described in sections 3.5 and 4.1.

4.3 Larger Infinity

   While on the subject of compatibility, there is one item which people
   have requested: increasing infinity.  The primary reason that this
   cannot be done is that it would violate backwards compatibility.  A
   larger infinity would obviously confuse older versions of rip.  At
   best, they would ignore the route as they would ignore a metric of
   16.  There was also a proposal to make the Metric a single octet and
   reuse the high three octets, but this would break any implementations
   which treat the metric as a 4-octet entity.

4.4 Addressless Links

   As in RIP-1, addressless links will not be supported by RIP-2.

5. Security Considerations

   The basic RIP protocol is not a secure protocol.  To bring RIP-2 in
   line with more modern routing protocols, an extensible authentication
   mechanism has been incorporated into the protocol enhancements.  This
   mechanism is described in sections 3.1 and 4.2.
















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RFC 1723                     RIP Version 2                 November 1994


Appendix A

   This is a simple example of the use of the next hop field in a rip
   entry.

      -----   -----   -----           -----   -----   -----
      |IR1|   |IR2|   |IR3|           |XR1|   |XR2|   |XR3|
      --+--   --+--   --+--           --+--   --+--   --+--
        |       |       |               |       |       |
      --+-------+-------+---------------+-------+-------+--
        <-------------RIP-2------------->

   Assume that IR1, IR2, and IR3 are all "internal" routers which are
   under one administration (e.g. a campus) which has elected to use
   RIP-2 as its IGP. XR1, XR2, and XR3, on the other hand, are under
   separate administration (e.g. a regional network, of which the campus
   is a member) and are using some other routing protocol (e.g. OSPF).
   XR1, XR2, and XR3 exchange routing information among themselves such
   that they know that the best routes to networks N1 and N2 are via
   XR1, to N3, N4, and N5 are via XR2, and to N6 and N7 are via XR3. By
   setting the Next Hop field correctly (to XR2 for N3/N4/N5, to XR3 for
   N6/N7), only XR1 need exchange RIP-2 routes with IR1/IR2/IR3 for
   routing to occur without additional hops through XR1. Without the
   Next Hop (for example, if RIP-1 were used) it would be necessary for
   XR2 and XR3 to also participate in the RIP-2 protocol to eliminate
   extra hops.

References

   [1] Hedrick, C., "Routing Information Protocol", STD 34, RFC 1058,
       Rutgers University, June 1988.

   [2] Malkin, G., "RIP Version 2 - Carrying Additional Information",
       RFC 1388, Xylogics, Inc., January 1993.

   [3] Malkin, G., and F. Baker, "RIP Version 2 MIB Extension", RFC
       1724, Xylogics, Inc., Cisco Systems, November 1994.

   [4] Malkin, G., "RIP Version 2 Protocol Analysis", RFC 1721,
       Xylogics, Inc., November 1994.

   [5] Malkin, G., "RIP Version 2 Protocol Applicability Statement", RFC
       1722, Xylogics, Inc., November 1994.








Malkin                                                          [Page 8]

RFC 1723                     RIP Version 2                 November 1994


Author's Address

   Gary Scott Malkin
   Xylogics, Inc.
   53 Third Avenue
   Burlington, MA 01803

   Phone:  [phone]
   EMail:  [e-mail]










































Malkin                                                          [Page 9]