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Network Working Group G. Malkin Request for Comments: 1722 Xylogics, Inc. Category: Standards Track November 1994

           RIP Version 2 Protocol Applicability Statement

Status 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.


 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

1. Protocol Documents

 The RIP-2 protocol analysis is documented in RFC 1721 [1].
 The RIP-2 protocol description is defined in RFC 1723 [2].  This memo
 obsoletes RFC 1388, which specifies an update to the "Routing
 Information Protocol" RFC 1058 (STD 34).
 The RIP-2 MIB description is defined in RFC 1724 [3].  This memo will
 obsolete RFC 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 in RFC 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 variable

Malkin [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 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:
  1. —- —– —– —–

|IR1| |IR2| |XR1| |XR2|

  1. -+– –+– –+– –+–

| | | |

  1. -+——-+————-+——-+–


 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 [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:
  1. —-


  1. —-

/ \

        /     \
    -----     -----     -----
    |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 1994

5. 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", RFC
     1724, 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.COM

Malkin [Page 5]

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