RFC 3627 (rfc3627) - Page 1 of 6

Use of /127 Prefix Length Between Routers Considered Harmful

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Network Working Group                                          P. Savola
Request for Comments: 3627                                     CSC/FUNET
Category: Informational                                   September 2003


      Use of /127 Prefix Length Between Routers Considered Harmful

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 (2003).  All Rights Reserved.

Abstract

   In some cases, the operational decision may be to use IPv6 /127
   prefix lengths, especially on point-to-point links between routers.
   Under certain situations, this may lead to one router claiming both
   addresses due to subnet-router anycast being implemented.  This
   document discusses the issue and offers a couple of solutions to the
   problem; nevertheless, /127 should be avoided between two routers.

1.  Introduction

   [ADDRARCH] defines Subnet-router anycast address: in a subnet prefix
   of n bits, the last 128-n bits are all zero.  It is meant to be in
   use of any one router in the subnet.

   Even though having prefix length longer than /64 is forbidden by
   [ADDRARCH] section 2.4 for non-000/3 unicast prefixes, using /127
   prefix length has gained a lot of operational popularity; it seems
   like that these prefix lengths are being used heavily in point-to-
   point links.  The operational practise has often been to use the
   least amount of address space especially in the presence of a large
   number of point-to-point links; it may be unlikely that all of these
   links would start to use /64's.  Using /127 has also other
   operational benefits: you always know which address the other end
   uses, and there is no "ping-pong" [PINGPONG] problem with older ICMP
   implementations (fixed now in [ICMPv3]).








Savola                       Informational
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