Anycast
Addressing technique in which the same IP prefix is announced from multiple locations, so traffic is routed to the topologically nearest one.
Anycast is an addressing and routing technique in which the same IP prefix is announced from multiple locations at the same time. Each location originates the prefix into BGP from its own ASN (or a shared one), and remote networks select whichever copy looks best according to their BGP decision process — usually the topologically nearest one. From the client's perspective there is a single destination address; from the network's perspective there are many independent servers behind it.
Anycast is widely used in core internet infrastructure. The DNS root servers and most large recursive and authoritative DNS services are anycasted so that queries terminate close to the user and survive the loss of individual sites. CDN edge networks use anycast to map users to a nearby point of presence without per-request routing logic. The same property makes anycast useful against volumetric DDoS: attack traffic is distributed across all sites announcing the prefix rather than concentrating on one.
Because path selection happens in BGP, anycast is best suited to stateless or short-lived request/response protocols (DNS over UDP, HTTP behind a load balancer at each site). Long-lived stateful sessions can be disrupted if a routing change moves the path from one site to another mid-flow, so anycast is normally combined with peering and careful prefix design rather than used as a generic load-balancing tool.
Key Points
- Same prefix originated from multiple sites via BGP
- Remote networks pick the topologically closest copy
- Heavily used by DNS root, recursive and authoritative services
- Common at CDN edges for low-latency content delivery
- Distributes volumetric attack traffic across all anycast sites
- Best suited to stateless or short-lived protocols