[afnog] IPv 6 Point to Point at /64?

Geert Jan de Groot GeertJan.deGroot at xs4all.nl
Wed Jun 6 16:26:12 UTC 2012

On Wed, 6 Jun 2012 15:04:49 +0200 =?ISO-8859-1?Q?I=F1igo_Ortiz_de_Urbina?= wrote:
> There was this document I cannot remember (RFC3627?), in which I read there
> were some concerns on using /127 for links which endpoints belong to
> different vendors. This was due, IIRC, to the possible differences in
> subnet-router anycast address treatment/implementation.

I think that's only part of the answer. Whether /64's or /127's are used,
the question to ask is: "do I know where packets to unused address space go".
Whether that's the remaining space of a /64 p-p, or the remaining
space of the /48 (/32?) behind it.

Suppose you have 2001:DB8:4242::/48. 
You start with the first subnet 2001:DB8:4242:1::/64.
No other subnets (yet).

Chances are that packets for 2001:DB8:4242:2::1 are sent 
to your network, but since you don't have the network 
2001:DB8:4242:2::/64, the default route sends them back, ad infinitum.
I see this happen much more often than one would expect.

The answer is to null-route unused space. Whether that's the remaining
of the /64 of the p-p link or the whole /48 behind it, null-route it
at the border.

Longer, more-specific routes will win over the null-route, so this is
configure and forget.

But, I think in this day of scarce IPv4 space, high utilisation and NAT, 
we have forgotten how to do routing. Sigh.

Geert Jan

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