What is anycast?

Anycast is a networking technique where the same IP address is assigned to multiple, geographically dispersed servers. When a user sends data to that IP, the internet’s routing system automatically directs the traffic to the nearest or best‑performing server, based on network distance and routing policies.

Let's break it down

  • You have one IP address (e.g., 203.0.113.1).
  • Several data centers around the world each configure a server to answer on that same IP.
  • Each data center advertises the IP to the internet using BGP (Border Gateway Protocol).
  • Routers on the internet see multiple routes to the same IP and pick the “closest” one-usually the one with the lowest latency or fewest hops.
  • Your request travels to that chosen server, which then replies, and the whole exchange looks just like a normal single‑server connection.

Why does it matter?

  • Speed: Users get faster responses because traffic goes to the nearest server.
  • Reliability: If one server or data center fails, traffic automatically shifts to the next closest server without any manual re‑configuration.
  • Scalability: A single IP can serve millions of users worldwide, simplifying DNS and client configuration.
  • Security: Anycast can absorb and disperse DDoS attacks across many locations, making it harder to overwhelm a single point.

Where is it used?

  • DNS root and top‑level domain servers (e.g., .com, .org) to provide fast, resilient name resolution.
  • Content Delivery Networks (CDNs) like Cloudflare, Akamai, and Fastly to deliver web assets quickly.
  • Cloud providers (AWS, Google Cloud, Azure) for services such as load balancers, API gateways, and edge caches.
  • DDoS mitigation services that route malicious traffic to scrubbing centers.
  • Global login or authentication services that need low latency for users everywhere.

Good things about it

  • Reduces latency for end users.
  • Increases uptime without complex failover scripts.
  • Simplifies DNS management-only one IP to publish.
  • Helps spread out traffic and attack load across many sites.
  • Works with existing internet routing protocols; no special client software needed.

Not-so-good things

  • Requires careful BGP configuration; mistakes can cause routing loops or blackholes.
  • Not ideal for stateful connections that need to stay on the same server (e.g., long‑running TCP sessions).
  • Debugging issues can be harder because traffic may be hitting any of several locations.
  • Some ISPs may have suboptimal routing, sending users to a farther node than expected.
  • Additional cost for operating multiple server sites and maintaining consistent software versions.