George, Let's take a clear example: Let's use 39/8 as the Bay area prefix. Suppose that mountainview.net gets 39.1.1/24 (just to be simple). Suppose that they happen to connect to Fix-west but not to the SF NAP. Suppose that sunnyvale.net gets 39.100.100/24 and connects to the NAP and not to Fix-west. Now, we can't summarize 39/8 into the backbones at either of the two locations. Consider what happens if a backbone with only 39/8 delivers a packet to the NAP for mountainview.net. What happens? In fact, we have to move the AAB out. And we have to at least subsume a biconnected (or triconnected?) subgraph of the backbones that contains the interconnects. The result is that we either summarize 39/8 to the backbones and someone provides free transit between the NAP and Fix-West, or we get no effective aggregation from 39/8. Tony