Anycast is a network addressing and routing technology. Its core concept lies in the "proximity" principle. That is to say, the same IP address is assigned to multiple server nodes that are geographically dispersed. During the data packet transmission process, the packets will be automatically routed to the node that is closest to the request initiator or has the optimal performance.
Tatyana Hammes
Apr 08, 2025
6 mins to read
1,The Basic Concept of Anycast
Anycast is a network addressing method. Its IP address does not uniquely correspond to a single terminal. Instead, it can be deployed on multiple servers, and each server uses the same Anycast IP address in the network. Routing protocols (such as BGP - Border GatewayProtocol) announce these IP addresses globally. In the network, data packets make routing decisions and select a path leading to the nearest (or the least costly) Anycast node.
Comparison with Other Addressing Methods:
Unicast: Each IP address uniquely corresponds to one terminal, and data packets are directly sent to this specified address.
Broadcast: Data is sent to all nodes in the network.
Multicast: Data packets are sent to a group of predefined subscribers.
Anycast: Data packets are sent to the "nearest" one among multiple nodes with the same address, usually through the nearest path dynamically calculated by the routing protocol.
2,The Working Principle of Anycast
1，Deploying the Same IP at Multiple Points In an Anycast architecture, a common Anycast IP is deployed in multiple geographical locations or networks. For example, DNS servers distributed globally or nodes in a content delivery network (CDN). Each node operates independently and, within its respective network area, announces the same IP to the upper-level network through routing protocols.
2，Path Decision Based on Routing Protocols A common implementation relies on routing protocols such as BGP. The steps are as follows:
Routing Announcement: Each Anycast node announces the routing information of having the Anycast IP address to its upstream autonomous system (AS). These announcements will spread along the Internet, enabling routers throughout the network to learn the multi-path information of this IP.
Routing Decision: When a data packet is sent from a client, the BGP routing decision mechanism between autonomous systems selects the path with the shortest distance or lowest cost based on various factors (such as AS path length, routing policies, link costs, etc.). Eventually, the data packet reaches a certain Anycast node along this optimal path.
Proximity Principle: Due to the "shortest path" or "lowest cost" selection strategy of BGP routing decisions, in most cases, data packets will be routed to the node that is physically closer to the sender, thus improving the response speed and reducing latency.
3，Dynamic Load and Failover Fault Tolerance: If an Anycast node fails to respond due to a fault or network anomaly, the relevant BGP routing information will be revoked or invalidated. At this time, routers throughout the network will automatically recalculate and select other reachable nodes, thus achieving seamless failover.
Traffic Dispersion: The routing selection at any moment is dynamically determined based on the network status. Therefore, even if multiple nodes use the same IP address, the traffic will usually be distributed to each node, naturally achieving load balancing.
3,Application Scenarios and Advantages
1,Typical Application Scenarios DNS System: The global root domain name server group widely adopts Anycast technology to ensure that query requests can be responded to quickly and have high availability. For example, by deploying root servers in multiple geographical regions, even if a single region fails, other regions can still respond to domain name resolution requests.
Content Delivery Network (CDN): CDNs use Anycast to direct user requests to the nearest cache server, thus reducing transmission latency, saving bandwidth, and enhancing the user experience.
Distributed Services and DDoS Protection: The multi-node deployment and traffic dispersion characteristics of Anycast can help balance large-scale traffic attacks and mitigate the impact of Distributed Denial of Service (DDoS) attacks. Attack traffic is automatically dispersed to multiple nodes, reducing the risk of a single node being under enormous pressure.
2,Advantages Reduced Latency: User requests are usually routed to the node that is physically or network-wise closest.
Increased Availability: When a certain node fails, BGP will automatically switch to other available nodes.
Natural Load Balancing: Traffic dispersion based on routing decisions reduces the load risk of a single node.
Scalability: Any application that requires the deployment of highly available services on a global scale can utilize Anycast technology to achieve expansion.
4,In-depth Technical Analysis
1,The Synergistic Effect between BGP and Anycast The Routing Characteristics of BGP: As the main routing protocol between autonomous systems, BGP allows each node to announce the same prefix. Anycast takes advantage of this feature and advertises the same prefix to adjacent networks at each deployment node. BGP determines the best exit route by comparing the AS path lengths, original metric values, and local routing policies of multiple possible paths. For example, on the Internet, different ISPs or data centers determine the "nearest" node based on the topological relationships between autonomous systems.
Path Uncertainty and Challenges: Although BGP is widely used, its "nearest" decision does not always completely conform to the physical geographical distance. The policies, business relationships, and routing policies between different autonomous systems may cause data packets not to take the absolutely shortest path during actual transmission. In addition, route flapping and frequent changes will also affect the performance of Anycast, especially in network environments where the topology changes frequently.
2,Geographical Distribution and Network Topology Multi-active Nodes: When deploying Anycast, multiple active nodes need to be set up in different geographical locations. This not only requires the deployment of hardware and the construction of cold data centers but also requires establishing stable interconnection relationships with major Internet exchange hubs.
The Complexity of Network Topology: Operating Anycast services on a global scale requires considering network supervision, policies, and commercial access issues among different countries.
Monitoring and Management: Operators need to use specialized tools to monitor the real-time status of BGP announcements, packet loss rates, and response latencies, and promptly respond to possible network anomalies or attacks.
3,The Limitations and Challenges of Anycast Uncertain Traffic Distribution: Since routing decisions rely on BGP's policies, the traffic distribution may sometimes be uneven, causing some nodes to bear excessive pressure.
Fault Detection: Although BGP can automatically revoke invalid routes, it cannot immediately adjust to the performance degradation of nodes (such as increased latency or a sharp increase in server load). Therefore, additional monitoring and load balancing measures are required.
The Complexity of Debugging and Troubleshooting: The uncertainty of multi-path routing makes troubleshooting and traffic statistics more complicated. Network engineers need to have an in-depth understanding of BGP routing and the interaction status between each node.
5,Conclusion
As an innovative network addressing and routing technology, Anycast greatly improves the response speed, fault tolerance, and global coverage capabilities of services by deploying the same IP address in different geographical locations and dynamically selecting the best path with the help of the BGP protocol. Although it still needs to face challenges such as uneven traffic distribution and complex routing policies in practical applications, its wide application in the global DNS system, CDN, and large-scale distributed services has already demonstrated the great value of Anycast. Understanding the working principle of Anycast and the underlying technology has important guiding significance for network architecture design and global service optimization.
