Use of /127 Prefix Length Between Routers Considered Harmful (RFC3627)
Original Publication Date: 2003-Sep-01
Included in the Prior Art Database: 2003-Sep-17
Internet Society Requests For Comment (RFCs)
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.
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 (C) The Internet Society (2003). All Rights Reserved.
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.
[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 [Page 1]
RFC 3627 /127 Prefix Length Considered Harmful September 2003
2. Scope of this Memo
This memo does not advocate the use of long prefixes, but brings up
problems for those that do want to use them, for one reason or
Detailed discussion on what is the "right" solution is out of the
scope; it is not the goal of this memo to try to find the "best"
addressing solution for everyone.
3. Problem with /127 and Two Routers
Note that this problem does not exist between a router and a host,
assuming the PREFIX::0/127 address is assigned to the router.
Using /127 can be especially harmful on a point-to-point link when
Subnet-router anycast address is implemented. Consider the following
sequence of events:
1. Router A and Router B are connected by a point-to-point link.
2. Neither has anything configured or set up on this link.
3. 3ffe:ffff::1/127 address is added to Router A; now it performs
Duplicate Address Detection (DAD) [NDISC] for 3ffe:ffff::1.
Router A also adds the Subnet-router anycast address
3ffe:ffff::0/127. (DAD is not performed for anycast addresses.)
4. Now Router B has been planned and configured to use
3ffe:ffff::0/127 as its unicast IPv6 address, but adding it will
fail DAD, and Router B does not have any address....