Using /126 for IPv6 router links

Mark Smith nanog at
Wed Jan 27 07:32:51 UTC 2010

On Wed, 27 Jan 2010 07:47:35 +0200 (EET)
Pekka Savola <pekkas at> wrote:

> On Tue, 26 Jan 2010, Igor Gashinsky wrote:
> > Matt meant "reserve/assign a /64 for each PtP link, but only configure the
> > first */127* of the link", as that's the only way to fully mitigate the
> > scanning-type attacks (with a /126, there is still the possibility of
> > ping-pong on a p-t-p interface) w/o using extensive ACLs..
> >
> > Anyways, that's what worked for us, and, as always, YMMV...
> That's still relying on the fact that your vendor won't implement 
> subnet-router anycast address and turn it on by default.  That would 
> mess up the first address of the link.  But I suppose those would be 
> pretty big ifs.

A minor data point to this, Linux looks to be implementing the
subnet-router anycast address when IPv6 forwarding is enabled, as it's
specifying Solicited-Node multicast address membership for the
all zeros node address in it's MLD announcements when an interface
comes up.

> -- 
> Pekka Savola                 "You each name yourselves king, yet the
> Netcore Oy                    kingdom bleeds."
> Systems. Networks. Security. -- George R.R. Martin: A Clash of Kings

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