> 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...
