RFC 3627 (rfc3627) - Page 2 of 6

Use of /127 Prefix Length Between Routers Considered Harmful

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.

   Similar scenarios also happen during router reboots, crashes and
   such.

   The usability of subnet-router anycast address between two routers on
   a point-to-point link is very questionable, but it is still a
   mandated feature of [ADDRARCH].  Workarounds for this are presented
   in the next section.

   As of yet, this kind of unexpected behavior hasn't been seen at large
   perhaps because the Subnet-router anycast address hasn't been
   implemented or too widely used.








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