[c-nsp] IPv6 subnets for point-to-point links

[sthaug at nethelp.no]

> >> Don't do it.  If you want to use shorter prefixes, I suggest doing
> >> something like /112 or whatever.
> >
> > Ah, but *why* should you not do it?
> 
> RFC 3627 gives some reasons why not.
> 
> In particular, subnet-router anycast address on /127 is ambiguous on a 
> link with two routers and can lead to serious problems.

The question is, why would you *want* anycast on point to point links?
According to RFC 3513,

   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 to which they have
   interfaces.

I see absolutely no reason why I would want this for my *own* use on
point to point links - and I also see no reason why somebody out on the
Internet should be able to use (e.g. ping) such anycast address on my
point to point links.

Operationally, /127 appear to work between Juniper routers in a quick
lab test.

(We use /126 on our point to point links. I'm not going to deploy /127
in our network anytime soon - for one thing I'd need to know quite a bit
more about how Cisco and Juniper react to /127 addresses. What I'm
arguing above is simply that the anycast functionality is both unneeded
and undesirable here.)

Steinar Haug, Nethelp consulting, sthaug at nethelp.no