Anycast Routing Mechanics
The Proximity of Identity
The Concept of One-to-Nearest
In traditional Unicast routing, there is a one-to-one mapping between an IP address and a physical interface. In Anycast, a single IP address is announced by multiple routers across the globe. When you send a request to an Anycast IP (like Google's `8.8.8.8`), the global routing table delivers your packet to the node that is topologically closest to you.
Note that "closest" does not always mean geographical distance—it means the path with the fewest BGP 'hops' or the lowest cost.
Anycast Global Resolver Map
The same IP (8.8.8.8) exists in 3 locations. Click a city to resolve.
Proximity-Based Latency Optimization
By distributing nodes geographically, Anycast significantly reduces the Propagation Delay. Instead of a request from Tokyo traveling all the way to a server in Virginia, it is intercepted by a Tokyo-based node sharing the same identity.
Challenges: Statefulness
Anycast is primarily used for stateless protocols like UDP (DNS) or short-lived TCP sessions. Because BGP can change routes mid-session (a 'flap'), a long-lived TCP connection might suddenly be routed to a different server that has no knowledge of the previous handshake.
Understanding Anycast is essential for designing High-Availability Systems that must withstand regional outages.