Simple definition
Anycast allows several devices or sites to advertise the same IP address so that routing normally sends each client to the instance considered best according to network metrics and policy.
Technical definition
Multiple nodes advertise the same prefix or address from different locations, often through a routing protocol such as BGP. The network then selects a route using its normal routing policy, so the chosen node is not necessarily the geographically closest one.
How it works / role
Each Anycast site advertises the same destination. If a site becomes unavailable, its route can be withdrawn and routing converges toward another instance. The service must tolerate a client being sent to a different node after a routing change.
What is it used for?
Distribute DNS, CDN, security, or other widely accessed services, reduce perceived latency, and improve geographic resilience.
Practical example
Several DNS resolvers share the same Anycast IP. A user in Europe is normally routed to a European instance, while a user in another region follows a different path to the same address.
Common issues
- Incorrect BGP advertisement or route leak
- Application state is not shared even though traffic can move between sites
- Asymmetric routing or a path change during a session
- Health checks keep advertising a site whose application service is unavailable
Key takeaway: Anycast distributes an address through routing; high availability also depends on application health, route withdrawal, and service design.