BGP

An Internet prefix may come out of a data center in Shanghai today and switch to Beijing or Hong Kong tomorrow. The same public address may follow very different paths for users on different carriers. A data center may already be healthy again, yet external traffic still detours around it. When you see this kind of behavior, the problem is usually no longer in one router’s forwarding table. It is higher up, in how autonomous systems tell each other which prefixes are reachable.

When the same service is deployed in many places, the usual approach is to give each node a different address and let DNS, a configuration center, or an upper-layer scheduler decide which one the user should connect to. That can work, but it has a clear cost: the access decision happens outside the network, so very often the name has already been resolved before the user discovers that they were sent somewhere suboptimal.