Thank you for comments. I know there are economic/contractual relationships between two networks, and BGP cannot find a path that the business rules forbid. But when in these cases, how to recover it? The network operators just wait for physically reparing the link or they may manully configure an alternative path by paying another network for transit service or finding a peering network?
It sounds like you are asking this question in the context of an Internet exchange point where you connect to the exchange point, and then negotiate separate peering agreements with each participant, or a telecom hotel/data centre. In the exchange point, you could theoretically have special "INSURANCE" peering agreements where you don't exchange traffic until there is an emergency, and then you can quickly turn it on, perhaps using an automated tool. In the data centre, you could theoretically have a similar sort of agreement that only requires cross-connect cables to be installed. In fact, you could already have the cross-connect cables in place, waiting to be plugged in on your end, or fully plugged in waiting for you to enable the port. I wonder if anyone on the list has such INSURANCE peering or transit arrangements in place? Given the fact that most providers will go to extra efforts to install new circuits when there is an emergency like the Taiwan quake, perhaps there isn't as much value to such insurance arrangements as you might think. If we ever get to the point where most circuit connections in the core are via switched wavelengths, then perhaps BGP will be used to find new paths when others have failed. --Michael Dillon