IPv6: numbering of point-to-point-links

Mon Jan 31 17:13:08 UTC 2011

> All of the (mostly religious) arguments about /64 versus any
> smaller subnets aside, I'm curious about why one would choose
> /126 over /127 for P-to-P links? Is this some kind of IPv4-think
> where the all-zeros and all-ones addresses are not usable
> unicast addresses? This isn't true in IPv6 (of course, it's not
> strictly true in IPv4 either). Is there another reason?

I setup a p2p /127 link and found that BGP would not peer over the link;
Changing to /126 resolved the problem. I never looked into it further
because I had intended to use /126 from the start. My guess is that
while BGP should be a unicast IP, Cisco's implementation uses an anycast
in some cases, disregarding the configured unicast address.

Just one practical example...