RFC 2091 (rfc2091) - Page 3 of 22

Triggered Extensions to RIP to Support Demand Circuits

RFC 2091                      Trigger RIP                   January 1997


   Periodic broadcasting by Bellman-Ford (distance vector) algorithm
   information broadcasting protocols IP RIP [1], IP RIP V2 [2] or
   Netware RIP and SAP [3] generally prevents WAN circuits from being
   closed.  Even on fixed point-to-point links the overhead of periodic
   transmission of RIP - and even more so SAP broadcasts - can seriously
   interrupt normal data transfer simply through the quantity of
   information which hits the line every 30 or 60 seconds.

   To overcome these limitations, this specification modifies the
   distance vector protocols so as to send information on the WAN only
   when there has been an update to the routing database OR a change in
   the reachability of a next hop router is indicated by the task which
   manages connections on the WAN.

   Because datagrams are not guaranteed to get through on all WAN media,
   an acknowledgement and retransmission system is required to provide
   reliability.

   The protocols run unmodified on Local Area Networks (LANs) and so
   interoperate transparently with implementations adhering to the
   original specifications.

   This proposal differs from Demand RIP [4] conceptually as follows:

   o  If a router has exchanged all routing information with its partner
      and some routing information subsequently changes only the changed
      information is sent to the partner.

   o  The receiver of routes is able to apply all changes immediately
      upon receiving information from a partner.

   These differences lead to further reduced routing traffic and also
   require less memory than Demand RIP [4].  Demand RIP also has an
   upper limit of 255 fragments in an update which is lifted in
   Triggered RIP (which does not use fragmentation).

2. Overview

   Multiprotocol routers are used on connection oriented Wide Area
   Networks (WANs), such as X.25 packet switched networks and ISDN
   networks, to interconnect LANs.  By using the multiplexing properties
   of the underlying WAN technology, several LANs can be interconnected
   simultaneously through a single physical interface on the router.








Meyer & Sherry              Standards Track