/127 between routers?

[Jim Burwell]

On 1/5/2010 08:34, Roger Jørgensen wrote:
> On tir, januar 5, 2010 15:38, Andrew Alston wrote:
>   
>> We use /127s all over the place on point to point links with no issues
>> (We went this route after considering what was said in the latest link
>> Janos posted below)
>>
>> Haven't really found any drawbacks with this yet
>>     
> Some year back I stopped using /127 when Linux refused to route my
> traffic. Had to add a static /128 for part of the /127 to get traffic
> flowing again. Later I changed to /126 and the problem disapeared. Wonder
> if I even had the same issue on some cisco routers to.
>
> Should be mention that it was all IPv6 in IPv4 tunnels so could be related
> to that to...
>
>
>   
Makes sense to me.  It would seem to me that /126s were always the
logical equivalents of /30s in the IPv4 world for use on p-t-p links. 
/127s always seemed "wrong"  to me, since it uses the "magic" all-zeros
network identifier address as a host address.  :p 

When I first started learning about IPv6, I wondered it the all-zeros or
all-ones host chunk of an IPv6 address held any special meaning as they
do in IPv4, and quickly figured out that all-ones doesn't, but all-zeros
still does.  A bit annoying in some ways since /127 would make a nice
XYZ::0,1 p-t-p interface address pairs, but oh well.  /126 it is.

Having said that, the existence of that RFC and draft worries me about
using /126s at all, and one wonders if just sticking with /64s as seems
to be extolled as the best practice is the "right thing".  Others have
also made good points about /64 for convenience and consistancy in IPv6
addressing plans.  If it's easy to get say, an entire separate /48 to
use for /64 tunnel end points, then there's not much point in using
/126s, even if the "wastefulness" bugs me a bit.  :)

I've also wondered about the wisdom of perhaps using site-unique
addresses for things like tunnel end points, similar to the way I've
used RFC1918s for the same purpose in the IPv4 world.  Obviously using
non-global-routables wouldn't be good for all circumstances, but for the
typical tunnel or p-t-p link where the interface address might only be
used as a gateway for a local route entry, would it be a bad idea?


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