Anycast
A network addressing and routing method in which a single IP address is shared by multiple devices in different locations.
Detailed Explanation
When a client sends a request to an Anycast IP, the network routes it to the 'closest' server (geographically or in terms of network hops). This is how CDNs and DNS providers achieve global scale and DDoS protection. If one location goes down, BGP automatically reroutes traffic to the next closest healthy location.
Quick Summary
Anycast advertises the same IP from many locations worldwide so users naturally hit the nearest healthy one. It's how CDNs, public DNS resolvers (1.1.1.1, 8.8.8.8), and edge networks deliver low latency and absorb DDoS attacks.
Key Takeaways
- BGP routes each client to the topologically closest instance, not necessarily geographically closest.
- If a location fails, BGP withdraws its route and clients drift to the next-closest location automatically.
- Massively distributes DDoS load, an attack from one region only hits the local instances.
- Works well for stateless or quickly-cacheable requests (DNS, static assets); harder for connection-affinity workloads.
- Provider-managed anycast (Cloudflare, Fastly, GCP, Route 53) is far easier than running your own BGP.
When to use it
- Public DNS resolvers and authoritative DNS providers.
- CDNs and edge networks delivering static and dynamic content.
- DDoS-resistant deployments where one IP must absorb global attack traffic.
- Global load balancing with automatic regional failover.
Common Mistakes
- Routing affinity assumptions; the same client might hit different instances if BGP recomputes.
- Storing connection state at a single anycast endpoint expecting affinity it doesn't guarantee.
- Confusing anycast (same IP, many locations) with unicast load balancing (different IPs, one logical service).
- Relying on anycast for latency wins without measuring; pathological BGP paths sometimes send traffic far.
Anycast, Frequently Asked
Anycast vs. GeoDNS?
GeoDNS returns different IPs based on the client's geographic resolver. Anycast returns one IP that the network routes to the nearest instance. Anycast generally has faster failover and is more accurate for the actual network path; GeoDNS has finer-grained control by region but is less reactive.
Can I run anycast for my own service?
Technically yes if you have your own ASN and IP space, but it's serious infrastructure work. Most teams get anycast benefits by putting their service behind Cloudflare, Fastly, or a cloud global load balancer.