RFC 2332 (rfc2332) - Page 2 of 52

NBMA Next Hop Resolution Protocol (NHRP)

RFC 2332                       NBMA NHRP                      April 1998


   Operation of NHRP as a means of establishing a transit path across an
   NBMA subnetwork between two routers will be addressed in a separate
   document (see [13]).

1. Introduction

   The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
   document, are to be interpreted as described in [15].

   The NBMA Next Hop Resolution Protocol (NHRP) allows a source station
   (a host or router), wishing to communicate over a Non-Broadcast,
   Multi-Access (NBMA) subnetwork, to determine the internetworking
   layer addresses and NBMA addresses of suitable "NBMA next hops"
   toward a destination station.  A subnetwork can be non-broadcast
   either because it technically doesn't support broadcasting (e.g., an
   X.25 subnetwork) or because broadcasting is not feasible for one
   reason or another (e.g., an SMDS multicast group or an extended
   Ethernet would be too large).  If the destination is connected to the
   NBMA subnetwork, then the NBMA next hop is the destination station
   itself.  Otherwise, the NBMA next hop is the egress router from the
   NBMA subnetwork that is "nearest" to the destination station.

   One way to model an NBMA network is by using the notion of logically
   independent IP subnets (LISs). LISs, as defined in [3] and [4], have
   the following properties:

      1)  All members of a LIS have the same IP network/subnet number
          and address mask.

      2)  All members of a LIS are directly connected to the same
          NBMA subnetwork.

      3)  All hosts and routers outside of the LIS are accessed via
          a router.

      4)  All members of a LIS access each other directly (without
          routers).

   Address resolution as described in [3] and [4] only resolves the next
   hop address if the destination station is a member of the same LIS as
   the source station; otherwise, the source station must forward
   packets to a router that is a member of multiple LIS's.  In multi-LIS








Luciani, et. al.            Standards Track