RFC 3627 (rfc3627) - Page 1 of 6
Use of /127 Prefix Length Between Routers Considered Harmful
Alternative Format: Original Text Document
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