RFC 2991 (rfc2991) - Page 1 of 9

Multipath Issues in Unicast and Multicast Next-Hop Selection

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Network Working Group                                           D. Thaler
Request for Comments: 2991                                      Microsoft
Category: Informational                                          C. Hopps
                                                     NextHop Technologies
                                                            November 2000


      Multipath Issues in Unicast and Multicast Next-Hop Selection

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

Abstract

   Various routing protocols, including Open Shortest Path First (OSPF)
   and Intermediate System to Intermediate System (ISIS), explicitly
   allow "Equal-Cost Multipath" (ECMP) routing.  Some router
   implementations also allow equal-cost multipath usage with RIP and
   other routing protocols.  The effect of multipath routing on a
   forwarder is that the forwarder potentially has several next-hops for
   any given destination and must use some method to choose which next-
   hop should be used for a given data packet.

1.  Introduction

   Various routing protocols, including OSPF and ISIS, explicitly allow
   "Equal-Cost Multipath" routing.  Some router implementations also
   allow equal-cost multipath usage with RIP and other routing
   protocols.  Using equal-cost multipath means that if multiple equal-
   cost routes to the same destination exist, they can be discovered and
   used to provide load balancing among redundant paths.

   The effect of multipath routing on a forwarder is that the forwarder
   potentially has several next-hops for any given destination and must
   use some method to choose which next-hop should be used for a given
   data packet.  This memo summarizes current practices, problems, and
   solutions.







Thaler & Hopps               Informational
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