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(IPng 4426) /126 or /127 -- neither!

Hi Pedro,
	Well if you mean by ununumbered ethernets assigning nodes
on the ethernet (which are peering in some arbitrary fashion)
in different ASs, addresses out of their respective AS's prefix then
the case is no different then the p-to-p link case.

In the ethernet case, as per Matt's suggestion you will configure
nodes from AS A on that etherenet with addresses from a global prefix
assigned to A and let nodes on the same subnet from AS B know that
this prefix is on-link though they are not configured with addresses
from the prefix and vice-versa.

Yes the question still remains which method is better. 
1) Do we use global prefixes assigned to the link which are not
   part of the prefixes assigned to either AS as you suggested
   or somebody did ?
2) Or configure nodes on the link with prefixes belonging to 
   their respective ASs as Matt suggest.
3) Or use a global prefix which is part of a prefix assigned
   to one of the ASs and configure all nodes on the link
   with that prefix. 

As you said in the cases 2) and 3) you have to reconfigure the
nodes with addresses out of new prefixes if renumbering occurs
and introducing them in the IGPs of the involved ASs.
But BGP anyway will require some reconfiguration on renumbering, 
atleast in policies.

While in case of using 1) you will need to setup an administrative

 > I don't understand why you are mixing the p-to-p issue with the peering
 > address issue. What if instead of a serial the peer uses an ethernet ?
 > Do you proprose to use unnumbered ethernets too ?
 >     Matt>   I believe routing protocols can perfectly well handle links whose
 >     Matt> endpoints have unrelated global-scope addresses.
 > In the context of BGP peering you have:
 > 	    i
 > 	A ----- B
 > if link is numbered (global scope address):
 > A and B will announce routes to each other using as nexthop the global
 > addresses Ia and Ib (respectivly). Those addresses are the ones annouced
 > in to the IBGP mesh. I's global prefix will be injected into the IGP.
 > if the link is unnumbered:
 > A and B will need to make those annoucements with Ag and Bg. That means
 > site B needs to manually configure a route to A's prefix and inject it
 > in it's own IGP a vice-versa.
 > So, taking the address out of the link only adds complexity and creates
 > two possible reasons for manual reconfiguration: the renumbering of
 > site A and the renumbering of site B. While you only had one with
 > a numbered link: the renumbering of link i.
 > I'm not claiming that using non-routable addresses on link i is the right
 > thing to do. I'm just trying to clarify what the potential problem might
 > be (i'm very tempted to say there is none)...
 >     Matt> RIPng, for example, uses link-local next-hop addresses
 > Which is a real pain in the neck... because of all the nice effects you
 > get when you try to redistribute RIP into/from other routing protocols.
 > But then you need to have link locals anyway in your table (because of
 > the potential need to send redirects)...
 >     Matt> In summary, the only capability that's lost by having no
 >     Matt> subnet allocated to a p-p link is the ability to address
 >     Matt> "either end of this link."
 > No. What you loose by not having a subnet in a p-to-p link is the
 > ability to address the link's end-points knowing only how to route
 > to the link. That is why numbered links are very useful.
 >     Matt>   The cost of providing that
 >     Matt> ability is to either use one of our very large /64 subnets
 >     Matt> per link,
 > Not really.
 >     Matt> or to do great violence to the interface
 >     Matt> identifier concept in the new addressing architecture.
 > Since i've absolutely no idea what interface identifiers are useful for i
 > cannot comment on that.
 >   Pedro.