Network Working Group J. Luciani
Request for Comments: 2332 Bay Networks
Category: Standards Track D. Katz
Core Competence, Inc.
NBMA Next Hop Resolution Protocol (NHRP)
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.
Copyright (C) The Internet Society (1998). All Rights Reserved.
This document describes the NBMA Next Hop Resolution Protocol (NHRP).
NHRP can be used by a source station (host or router) connected to a
Non-Broadcast, Multi-Access (NBMA) subnetwork to determine the
internetworking layer address and NBMA subnetwork addresses of the
"NBMA next hop" towards a destination station. 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. NHRP is intended for use in a multiprotocol
internetworking layer environment over NBMA subnetworks.
Note that while this protocol was developed for use with NBMA
subnetworks, it is possible, if not likely, that it will be applied
to BMA subnetworks as well. However, this usage of NHRP is for
This document is intended to be a functional superset of the NBMA
Address Resolution Protocol (NARP) documented in .
Luciani, et. al. Standards Track [Page 1]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 ).
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 .
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  and , 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
3) All hosts and routers outside of the LIS are accessed via
4) All members of a LIS access each other directly (without
Address resolution as described in  and  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 [Page 2]RFC 2332 NBMA NHRP April 1998
configurations, hop-by-hop address resolution may not be sufficient