Imagine you could assign the exact same IP address to servers in New York, London, Tokyo, and São Paulo—and when users connect to that address, the network automatically routes them to the nearest server. No DNS lookups to update, no complex routing policies to maintain, no geographic databases to query. The network itself makes the optimal routing decision, transparently and instantly.

This isn't science fiction—it's Anycast routing, a networking technique that has become foundational to the modern internet. Every time you use Cloudflare's 1.1.1.1 DNS resolver, access content from a major CDN, or connect to Google's services, you're benefiting from Anycast. It's the invisible infrastructure enabling sub-100ms response times worldwide.

Understanding Anycast is essential for system designers building globally distributed services, as it represents a fundamentally different approach to traffic distribution than DNS-based load balancing.

To understand Anycast, we must first contrast it with other IP addressing schemes:

Unicast (One-to-One): The standard addressing model. Each IP address identifies exactly one destination. When you send a packet to 192.0.2.1, it goes to the single server assigned that address.

Broadcast (One-to-All): A packet sent to a broadcast address (e.g., 255.255.255.255 on a local network) is delivered to all hosts on that network segment.

Multicast (One-to-Many): Packets sent to a multicast address are delivered to all hosts that have joined that multicast group. Used for streaming video and IPTV.

Anycast (One-to-Nearest): The same IP address is assigned to multiple hosts at different network locations. Packets are routed to the topologically 'nearest' host based on BGP routing metrics. The network selects the destination, not the application.

How Anycast Works at the Network Layer:

Anycast operates through the Border Gateway Protocol (BGP), the routing protocol that governs how packets traverse the internet between autonomous systems (ASes).

Here's the mechanism:

1. Identical Address Announcement: Multiple servers in different physical locations announce the same IP address (or IP prefix) to their upstream BGP peers.

2. BGP Path Selection: Each router on the internet receives multiple BGP announcements for the same prefix. It selects the 'best' path based on BGP metrics: shortest AS path, lowest origin type, lowest MED, etc.

3. Nearest Server Receives Traffic: When a client sends a packet to the Anycast address, it follows the 'best' BGP path, naturally reaching the topologically nearest server.

4. Automatic Failover: If a server goes down, it stops announcing the route. BGP converges (within seconds to minutes), and traffic automatically routes to the next-nearest server.

Anycast offers unique advantages that make it indispensable for certain use cases. Understanding these advantages—and their limitations—helps architects select the right tool for global traffic distribution.

Advantage 1: Zero-Configuration Geographic Routing

With DNS-based GSLB, you must maintain geographic databases, health checks, and routing policies. A user in Sydney requires your DNS server to recognize their location and return the appropriate IP. With Anycast, none of this is necessary—the network itself handles geographic routing based on real-time topology.

Advantage 2: Resilience Through Automatic Failover

When an Anycast node fails, it stops announcing its BGP route. Upstream routers detect the route withdrawal and BGP reconverges, typically within 30-90 seconds. Traffic automatically flows to the next-nearest healthy node with no DNS TTL delays, no health check propagation, no manual intervention.

Advantage 3: DDoS Attack Absorption

Anycast is a powerful DDoS mitigation strategy. Attack traffic targeting an Anycast address is distributed across all announcing nodes globally. A 100 Gbps attack becomes 10 Gbps distributed across 10 points of presence—each absorbing a fraction of the attack while legitimate traffic continues flowing.

Real-World Anycast Deployments:

Anycast is particularly well-suited for stateless, request-response protocols where each packet is self-contained and doesn't require maintaining a session across multiple exchanges. DNS over UDP is the canonical example.

Why DNS and Anycast Are a Perfect Match:

A DNS query is typically a single UDP packet (the query) followed by a single UDP response (the answer). There's no connection state to maintain, no session to preserve, and each query is independent. If the Anycast node changes between queries (due to BGP reconvergence), it doesn't matter—each query is handled completely by whichever node receives it.

The Anycast DNS Flow:

Other Stateless Use Cases:

Beyond DNS, Anycast works well for:

• NTP (Network Time Protocol): Time synchronization queries are stateless.
• RADIUS/TACACS+: Authentication protocols with request-response patterns.
• Syslog over UDP: Log shipping doesn't require connection state.
• QUIC Initial Handshake: QUIC's connection migration properties make it Anycast-friendly (discussed later).
• WebRTC STUN/TURN: Session setup protocols for real-time communication.

Anycast for DDoS Mitigation Services:

DDoS mitigation platforms use Anycast extensively. When attack traffic targets an Anycast IP:

1. Traffic is distributed globally based on attacker geography.
2. Each PoP handles a fraction of attack traffic.
3. Scrubbing centers filter malicious traffic at the edge.
4. Clean traffic is forwarded to origin servers.
5. No single point experiences the full attack volume.

While Anycast works seamlessly for stateless UDP protocols, TCP presents challenges due to its stateful, connection-oriented nature. Understanding these challenges—and their solutions—is critical for deploying Anycast in production HTTP/HTTPS services.

The TCP State Problem:

TCP connections maintain state on both client and server: sequence numbers, acknowledgments, window sizes. This state is tied to a specific server. If BGP routing changes during an active TCP connection (called a 'flap'), packets may route to a different Anycast node that has no knowledge of the connection—causing a connection reset.

When BGP Flaps Happen:

Impact on TCP Connections:

When a BGP flap causes packets to route to a different PoP:

1. The new PoP receives TCP packets for an unknown connection.
2. The PoP sends TCP RST (reset), terminating the connection.
3. The client sees a connection error and must reconnect.
4. If the flap resolves quickly, subsequent packets may return to the original PoP—but the connection is already broken.

For short-lived HTTP/1.1 or HTTP/2 connections that complete in seconds, this is usually acceptable—the probability of a flap during a single request is low. For long-lived connections (WebSockets, streaming, gRPC streams), the risk increases significantly.

Solutions for TCP over Anycast:

Implementing Anycast requires BGP capabilities, IP address resources, and careful network engineering. The complexity of implementation varies significantly between cloud and on-premise environments.

Requirements for Anycast:

Cloud Provider Anycast Options:

Most organizations don't operate their own BGP infrastructure. Cloud providers offer managed Anycast or Anycast-like services:

Self-Operated Anycast (Advanced):

For organizations requiring full control, implementing Anycast involves:

1. Obtain IP Resources: Apply for a /24 (minimum BGP-routable) from your Regional Internet Registry.
2. Obtain ASN: Apply for an Autonomous System Number.
3. Establish BGP Peering: Connect to upstream providers or Internet Exchanges at each PoP location.
4. Configure BGP Announcements: Announce the same prefix from all locations with appropriate BGP attributes.
5. Implement Health-Based Withdrawal: Automatically stop announcing routes when the local service is unhealthy.
6. Monitor BGP Propagation: Use looking glass servers and BGP monitoring tools to verify proper propagation.

With both DNS-based load balancing and Anycast available for global traffic distribution, when should you use each? Understanding the tradeoffs helps you architect optimal solutions.

Fundamental Differences:

When to Choose Anycast:

• Stateless UDP protocols: DNS, NTP, STUN—Anycast is ideal.
• DDoS mitigation: Anycast's distributed absorption is essential.
• Ultra-low latency requirements: BGP routing is faster than DNS resolution.
• Simplicity at scale: Operating hundreds of PoPs with per-PoP unicast IPs is operationally prohibitive.

When to Choose DNS-Based GSLB:

• Complex routing policies: Weighted, geolocation, latency, failover chains.
• Application-aware routing: Different backends for different user segments.
• TCP/HTTP services: Where connection state matters and flaps are unacceptable.
• Hybrid cloud routing: Directing traffic between cloud providers based on policies.

The Hybrid Approach:

Most sophisticated global architectures combine both techniques:

1. Anycast at the Edge: CDN/edge network uses Anycast to route users to nearest PoP.
2. DNS-Based Origin Selection: GSLB directs traffic to appropriate origin data centers.
3. Application-Layer Routing: Within data centers, application load balancers make final routing decisions.

Anycast routing represents a powerful network-layer technique for distributing traffic globally. It's the invisible backbone enabling the performance and resilience of the internet's largest services. Let's consolidate the key concepts:

What's Next:

We've covered GSLB, DNS-based load balancing, and Anycast routing. Next, we'll explore GeoDNS—a specialized application of DNS-based routing that enables precise geographic targeting for regulatory compliance, localized content, and regional service differentiation.