Well, first I think we need to agree that there are two different cases here:
1) interconnecting IXes operated by the same party, vs.
2) interconnecting IXes operated by different parties.
PAIX has successful implementations of both of these (I count our
metro strategy as an instance of the first sort, and our varied
interconnections to other switch fabrics operated by other people as
an instance of the second). With both, as I stated earlier, a thorough
understanding of the shortcomings of L2 fabrics as a place for
different parties to meet is crucial to avoiding service-affecting
outages. The lessons learned along the way are very helpful in
debugging quickly and thoroughly when a service-affecting outage