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What is Anycast?
- Authors
- Name
- Alex Lee
- @alexjoelee
Anycast is a network routing technique used to direct traffic to the nearest or most optimal destination in a group of potential destinations that share the same IP address. Unlike traditional unicast, which sends traffic from one sender to a specific receiver, or multicast, which targets multiple specific receivers, anycast allows multiple servers (or network nodes) to share the same IP address, with the network automatically routing traffic to the closest or best-performing instance.
How Anycast Works
Several geographically dispersed servers are configured with the same IP address, typically across different locations or data centers.
When a user or client sends a request to an anycasted IP address, the routing protocol (typically BGP—Border Gateway Protocol) directs the request to the nearest or best-performing server based on the routing information available. The "nearest" server is usually determined by network distance or lowest latency.
The participating servers advertise the same IP address via BGP to the internet. The BGP routers at various points in the network select the route that has the shortest path (or best routing metric) to the user based on the BGP announcements. As a result, the traffic is directed to the server closest to the user, often reducing latency and improving response times.
One of the key benefits of anycast is resilience. If one server goes down or becomes unreachable, BGP can dynamically reroute traffic to the next closest server. This feature enhances availability and provides failover capabilities, ensuring uninterrupted service even if one or more nodes are unavailable.
Key Uses of Anycast
DNS Resolution
Anycast is widely used for DNS. This helps reduce latency in name resolution and also enhances redundancy. If one DNS server becomes unreachable, other servers in the anycast group can handle the request.
CDNs like us use anycast to deliver content to end-users from the closest cache server. This minimizes latency and delivers the content faster, improving user experience.
DDoS Mitigation
Anycast can help mitigate large-scale DDoS (Distributed Denial of Service) attacks. In the event of a DDoS attack on one server, traffic can be rerouted to other servers, preventing a single point of failure and maintaining service availability for real users.
Global Load Balancing:
Anycast enables global load balancing across multiple locations. By directing traffic to the most optimal (or geographically closest) server, anycast can distribute traffic efficiently, improving response time and reducing strain on any single server.