Anycast is a network addressing and routing scheme where a single IP address is assigned to multiple physical nodes distributed globally. Unlike Unicast, which targets a specific destination, Anycast routes traffic to the "closest" available node based on BGP (Border Gateway Protocol) path metrics. This mechanism handles load distribution at the network layer rather than through application-level balancing.
How it works
Routers in various data centers announce the same IP prefix to the internet. When a client sends a packet, the network infrastructure selects the path with the shortest AS path or lowest routing cost. If a specific site becomes unavailable, its BGP announcement is withdrawn, and traffic automatically reroutes to the next optimal destination without any configuration changes on the client side.
Primary use cases include:
- Latency reduction by serving requests from the network edge.
- DDoS mitigation: volumetric attacks are dispersed across multiple points of presence (PoPs), preventing a single site from being overwhelmed.
- High availability for global infrastructure services.
Anycast is fundamental for Root DNS servers and Content Delivery Networks (CDNs). For instance, Cloudflare uses Anycast to serve traffic from over 300 cities using the same IP space, which effectively localizes traffic and provides inherent redundancy for over 20% of the web.