With anycast, the same IP address can be assigned to n servers, potentially distributed across the world. The internet’s core routing protocol, BGP, would then automatically route packets from the source to the closest (in hops) server. For example, in the following illustration, if a user Bob wants to connect to an anycast IP 18.104.22.168, its packets would be routed to PoP A because A is only 3 hops away from Bob, while all other PoPs are 4 hops or more.
Anycast’s property of automatically routing to the closest PoP gives us an easy solution to our PoP assignment problem. If LinkedIn assigned the same IP to all its PoPs, then:
- We would not need to rely on DNS-based geographical assignments (DNS would just hand out that one IP)
- None of the problems associated with DNS-based PoP assignments would arise
- Our users would get routed to the closest PoP automatically…