Use of IPv6 /127 prefixes

Mike Leber mleber at he.net
Tue May 1 01:00:23 UTC 2001


I've run into an interesting question regarding the widespread practice of
using /127 prefixes under IPv6.  I'm posting this to nanog instead of
6bone because I want to ask nanog IPv6 people.

Allocating a /127 is a common convention under IPv6 to provide globally
routable addresses for a point to point connection, such as a tunnel. Many
tunnel brokers use it, etc. I know that its use is widespread on the links
between cisco routers at the core the IPv6 Internet without obvious
trouble.

However, it would appear to be in conflict with anycast addresses (btw,
anycast addresses are not the same as IPv4 broadcast addresses).  RFC 2373
section 2.6.1 regarding "Required Anycast Address":

  The "subnet prefix" in an anycast address is the prefix which
  identifies a specific link. This anycast address is syntactically
  the same as a unicast address for an interface on the link with the
  interface identifier set to zero.

  Packets sent to the Subnet-Router anycast address will be delivered
  to one router on the subnet. All routers are required to support the
  Subnet-Router anycast addresses for the subnets which they have
  interfaces.

So what is anycast used for?

I've see lots of links where the the addresses used are
NNNN:NNNN:NNNN:NNNN:NNNN:NNNN:NNNN:000E/127 and
NNNN:NNNN:NNNN:NNNN:NNNN:NNNN:NNNN:000F/127 which works, yet shouldn't if
the 000E address is allways anycast on both sides of the link.

Why do /127s work for point to point links?

Mike.

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