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Use of /127 Prefix Length Between Routers Considered Harmful (RFC3627)

IP.com Disclosure Number: IPCOM000019489D
Original Publication Date: 2003-Sep-01
Included in the Prior Art Database: 2003-Sep-17
Document File: 7 page(s) / 12K

Publishing Venue

Internet Society Requests For Comment (RFCs)

Related People

P. Savola: AUTHOR

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.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 27% of the total text.

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 [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

another.

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....