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Network Working Group                                          G. MalkinRequest for Comments: 1722                                Xylogics, Inc.Category: Standards Track                                  November 1994RIP Version 2 Protocol Applicability StatementStatus of this Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Abstract   As required by Routing Protocol Criteria (RFC 1264), this report   defines the applicability of the RIP-2 protocol within the Internet.   This report is a prerequisite to advancing RIP-2 on the standards   track.1.  Protocol Documents   The RIP-2 protocol analysis is documented inRFC 1721 [1].   The RIP-2 protocol description is defined inRFC 1723 [2].  This memo   obsoletesRFC 1388, which specifies an update to the "Routing   Information Protocol"RFC 1058 (STD 34).   The RIP-2 MIB description is defined inRFC 1724 [3].  This memo will   obsoleteRFC 1389.2.  Introduction   This report describes how RIP-2 may be useful within the Internet.   In essence, the environments in which RIP-2 is the IGP of choice is a   superset of the environments in which RIP-1, as defined inRFC 1058   [1], has traditionally been used.  It is important to remember that   RIP-2 is an extension to RIP-1; RIP-2 is not a new protocol.  Thus,   the operational aspects of distance-vector routing protocols, and   RIP-1 in particular, within an autonomous system are well understood.   It should be noted that RIP-2 is not intended to be a substitute for   OSPF in large autonomous systems; the restrictions on AS diameter and   complexity which applied to RIP-1 also apply to RIP-2.  Rather, RIP-2   allows the smaller, simpler, distance-vector protocol to be used in   environments which require authentication or the use of variableMalkin                                                          [Page 1]

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 Applicability3.1 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 itMalkin                                                          [Page 2]

RFC 1722                  RIP-2 Applicability              November 1994   as a next hop, even if it is a better next hop than XR1.  Of course,   XR1 knows this and can indicate, via the Next Hop field, that XR2 is   the better next hop for some routes.   Another use for Next Hop has also been found.  Consider the following   example topology:           -----           |COR|           -----           /   \          /     \      -----     -----     -----      |RO1|-----|RO2|=====| R |      -----     -----     -----   The three links between the Central Office Router (COR) and the   Remote Office routers (RO1 and RO2) are all Dial-On-Demand (DOD)   links.  The link between RO2 and R is a fixed link.  Once all of the   routers have been initialized, the only routes they know about are   the configured static routes for the DOD links.  Assume that   connections between COR and RO1, and COR and RO2 are established and   RIP information is passing between the routers.  RO1 will ignore   COR's route to RO2 because it already has a better one; however, it   will learn to reach R via COR.   If we assume that RO1 and RO2 are only capable of establishing one   link at a time, then RO1 will not be able to reach RO2; however, RO1   will be able to reach R.  Worse still, if we assume that traffic   stops and the DOD links drop due to inactivity, an attempt by RO1 to   reach R will trigger the dialing of two links (through COR).  Of   course, once RO1 establishes a link to RO2, the problem corrects   itself because the new route to R is one hop shorter.   To correct this problem, the routers may use the Next Hop field to   indicate their next hop.  Consider the following route advertisements   during the period described above (before the RO1/RO2 link has ever   been established):      Sender  Recvr   Route   NextHop  Metric      =======================================      RO2     COR     R       0        1      ---------------------------------------      COR     RO1     RO2     0        1                      R       RO2      2      ---------------------------------------Malkin                                                          [Page 3]

RFC 1722                  RIP-2 Applicability              November 1994   When R01 receives the two routes from COR, it will ignore the route   for RO2, as mentioned above.  However, since R is not in RO1's   routing table, it will add it using a next hop of RO2 (because RO2 is   directly connected, after a fashion).  Note that COR does count   itself in R's metric; this is less than accurate, but entirely safe   and correctable (when the RO1/RO2 link comes up).  Suppose, now, that   the RO1/RO2 link did not exist.  RO1 would ignore the specification   of RO2 as the next hop to R and use COR, as it would if no Next Hop   had been specified.   Note that this is not a recursive algorithm; it only works to   eliminate a single extra hop from the path.  There are methods by   which this mechanism might be extended to include larger   optimizations, but the potential to create routing loops has not been   sufficiently analyzed to specify them here.3.3 Authentication   The need for authentication in a routing protocol is obvious.  It is   not usually important to conceal the information in the routing   messages, but it is essential to prevent the insertion of bogus   routing information into the routers.  So, while the authentication   mechanism specified in RIP-2 is less than ideal, it does prevent   anyone who cannot directly access the network (i.e., someone who   cannot sniff the routing packets to determine the password) from   inserting bogus routing information.   However, the specification does allow for additional types of   authentication to be incorporated into the protocol.  Unfortunately,   because of the original format of RIP packets, the amount of space   available for providing authentication information is only 16 octets.3.4 Multicasting   The RIP-2 protocol provides for the IP multicasting of periodic   advertisements.  This feature was added to decrease the load on   systems which do not support RIP-2.  It also provides a mechanism   whereby RIP-1 routers will never receive RIP-2 routes.  This is a   feature when correct use of an advertised route depends on knowing   the precise subnet mask, which would be ignored by a RIP-1 router.4.  Conclusion   Because the basic protocol is unchanged, RIP-2 is as correct a   routing protocol as RIP-1.  The enhancements make RIP-2 useful in   environments which RIP-1 could not handle, but which do not   necessitate the use of OSPF by virtue of requirements which RIP-2   does not satisfy.Malkin                                                          [Page 4]

RFC 1722                  RIP-2 Applicability              November 19945.  References   [1] Malkin, G., "RIP Version 2 Protocol Analysis",RFC 1721,       Xylogics, Inc., November 1994.   [2] Malkin, G., "RIP Version 2 - Carrying Additional Information",RFC 1723, Xylogics, Inc., November 1994.   [3] Malkin, G., and F. Baker, "RIP Version 2 MIB Extension",RFC1724, Xylogics, Inc., Cisco Systems, November 1994.6.  Security Considerations   Security issues are not discussed in this memo.7.  Author's Address   Gary Scott Malkin   Xylogics, Inc.   53 Third Avenue   Burlington, MA 01803   Phone:  (617) 272-8140   EMail:  gmalkin@Xylogics.COMMalkin                                                          [Page 5]