I believe that Vince is completely correct. With current routing protocols the NSFNET AUP policy can only be implemented with contorted and sub-optimal topologys and routing configurations. (One could argue that the policys are optimal for securing other goals in a wider arena, but that is not a technical discussion). In a general sense PC routing (that is "politically correct routing") requires switches to know the usage policys of both the source and destination sites. This means that traffic must be routed on the basis of both its destination address and SOURCE address and there needs to be some mechanism of associating the usage policys with remote addresses. Neither part of this can be accurately implemented by current protocols and architectures. Consider the following simpler situation that came up in Pennsylvania a while back: We (PSCnet) are NSF R&E. PREPnet transits PSCnet to reach the NSFnet backbones. Intra PREPnet traffic is NOT subject to the NSF usage rules, and does not distinguish between commercial and non-commercial internal sites. There was a proposal for PREPnet to acquire an additional ANS connection to address two goals: redundant paths for the research users and an external path that was not subject to the NSF rules. It was correctly observed that inbound traffic (from the backbones to the sites) could be PC routed, as long as the remote site/backbone/interchange did the correct thing. However, outbound traffic (sites to backbones) could not be "PC routed". The problem is that all traffic from commercial sites to remote commercial sites MUST leave via the ANS connection, yet all traffic from research sites strongly prefers to leave via the PSCnet connection. Given the topology under consideration this required traffic to flow in opposite directions on the same link to the same destination, depending on the source of the traffic. This can not be done today. period. PSC's position was that if PREPnet accepted comercial interstate traffic from any customers, then PSCnet could not accept any traffic from PREPnet. PREPnet would then be single attached via ANS. Any other position would have put us in violation of our funding. (This predated the NSF/ANS co+re policy, which provides an out.) As I look over the other replys, I see that many have missed a point that I assumed: The problems arise when there is a (complex) midlevel carrying mixed traffic between assorted sites and both flavors of backbones. It is not a problem if the midlevel is "pure" research or commercial. It is less of a problem if both backbones land on the same FDDI dmz. It is clear to me that one of two things is going to happen: The rules will change, and there will be good technical solutions. -or- The rules will not change, and we will have split research and commercial networks with weak interconnects. Organizations that really need to function in both worlds will have two network numbers or two connections.... --MM--