BGP Anycast and Traffic Filtering: How Distributed DDoS Protection Works

During a powerful DDoS attack, servers may remain fully operational, yet users still lose access to the site. The reason is straightforward. Malicious packets overwhelm the communication channel or overload network equipment before they reach the application. Increasing server capacity in this situation is practically useless.

One protection method involves distributing incoming traffic among several geographically dispersed scrubbing centers. BGP Anycast technology makes this possible by allowing requests to reach different sites through identical IP addresses. Combined with filtering systems, this architecture helps mitigate large-scale attacks while preserving the availability of protected resources.

What is BGP Anycast and How Routing Works

In ordinary routing, an IP address leads to a specific network. Anycast makes the same IP address reachable via multiple nodes located in different geographic points. A user connects to the familiar address, and routers decide which site should receive the packets.

Suppose a company places traffic processing centers in Moscow, Frankfurt, and Singapore. All three sites accept connections on one public IP address. A user from Russia will most likely reach the Moscow node, one from Germany the Frankfurt node, and one from Southeast Asia the Singapore node. Visitors do not need to know about the existence of multiple servers.

Selection of the path is handled by BGP (Border Gateway Protocol), the protocol for exchanging routing information between autonomous systems on the internet. Operators announce the availability of one IP prefix from different points, after which routers determine the preferred route based on network topology and configured policies.

BGP does not measure geographic distance and does not guarantee selection of the server with the lowest latency. Therefore, traffic from Moscow may sometimes travel through a European site even when the Moscow center is closer. Load distribution is also not necessarily uniform.

Nevertheless, Anycast provides several useful capabilities:

• Traffic distribution. Multiple sites simultaneously accept requests for one service.
• Improved fault tolerance. When a node becomes unavailable, routes can be rebuilt through other centers.
• Reduced latency. Well-placed sites and good connectivity with operators allow users to be served closer to the source of requests.
• Scalability. New nodes can be added without changing IP addresses used by clients.

Anycast determines the network entry point but does not replace a load balancer. Distribution of requests among servers inside the chosen site is handled by separate mechanisms.

How Anycast Helps Mitigate DDoS Attacks

The main advantage of distributed protection lies in the ability to receive malicious traffic at multiple sites instead of one. Consider an attack involving tens of thousands of compromised devices from Europe, Asia, and America. With suitable network topology, packets will arrive at different regional scrubbing centers rather than converging at a single point.

Each center processes the incoming flow, separates suspicious packets from legitimate traffic, and forwards the latter to the customer’s infrastructure. As a result, it becomes harder for the attacker to overload one channel or server, and the provider can utilize resources from several sites.

A typical processing scheme works as follows:

• Reception. BGP directs the incoming flow to one of the available Anycast nodes.
• Analysis. The system checks packet parameters, number of connections, request intensity, and signs of network anomalies.
• Filtering. Malicious packets are dropped, while suspicious flows receive rate limits or additional verification.
• Delivery. Legitimate traffic is sent to protected servers via the provider’s backbone, a direct connection, or a network tunnel.

In many implementations, filtering occurs directly at the Anycast site. There is no need to first accept packets at one node and then forward the entire stream to a separate scrubbing center. The earlier malicious traffic is discarded, the fewer resources are spent on its transmission.

Types of Attacks Stopped by Scrubbing Centers

DDoS attacks differ in their mechanism of impact. Some overload communication channels, others exhaust network equipment resources, and still others target applications directly. Modern scrubbing centers therefore combine several detection and filtering methods.

Common examples of network and volumetric attacks include UDP Flood, SYN Flood, ICMP Flood, and reflection/amplification attacks. The attacker sends a huge number of packets directly or forces third-party servers to generate responses toward the victim.

Protection involves protocol parameter validation, packet rate limiting, TCP connection control, and blocking of characteristic malicious flows. For SYN Flood, the system can verify correct connection establishment, preventing the attacker from exhausting equipment resources with many incomplete sessions.

Application-layer attacks on web applications and APIs may not require massive data volumes. The attacker sends many outwardly correct HTTPS requests to resource-intensive functions. The channel remains free, but servers gradually fail to handle the load. In such cases, WAF solutions, request rate limiting, behavioral analysis, and additional client checks are used.

Fault Tolerance and Limitations of Anycast

The distributed architecture reduces dependence on any single traffic processing center. If a site loses network connectivity and stops announcing routes via BGP, packets can shift to other available nodes. However, BGP does not independently verify application health. Operators therefore link service health checks with route management mechanisms.

Additional limitations must be considered when designing protection:

• Uneven load distribution
• Regional attacks targeting specific nodes
• Overly aggressive filtering rules
• Bottlenecks on delivery channels after scrubbing
• Bypass via direct access to unprotected origin IPs

To prevent bypass, access to the original infrastructure is restricted to trusted addresses of the protection provider or closed communication channels are used.

Practical Implementation and Provider Selection

Large providers combine scrubbing centers into a unified infrastructure with shared routing, filtering, and monitoring systems. Qrator Labs operates a distributed network of scrubbing centers within a single autonomous system. Akamai Prolexic uses a similar anycast approach across a global protective infrastructure.

Two main connection models exist: always-on filtering and on-demand protection. Hybrid architectures are also possible. For ordinary websites, a reverse proxy with distributed points of presence is often sufficient. Organizations with their own IP subnets may require BGP-level protection using tunnels such as GRE.

When choosing a provider, key criteria include geography and connectivity, per-site performance in bits and packets per second, reaction speed, application-layer protection capabilities, management of blocks, fault tolerance, and monitoring features.