What is anycast routing?
Why Interviewers Ask This
Advanced questions like this reveal whether a candidate has internalized Networking deeply enough to make architectural decisions. Strong answers demonstrate both breadth and depth of experience.
Answer
Anycast is a network addressing and routing methodology where traffic is routed to the topologically nearest node in a group of potential receivers, all sharing the same IP address. The routing infrastructure (BGP) determines which node is "closest" based on routing metrics. This differs from unicast (one-to-one), multicast (one-to-many-interested), and broadcast (one-to-all). Anycast applications: DNS root servers — there are only 13 root server IP addresses but hundreds of physical servers worldwide all sharing those IPs; queries go to the nearest server. CDN — Cloudflare and Fastly use anycast to route requests to the nearest edge PoP. DDoS mitigation — anycast distributes attack traffic across multiple PoPs. DNS anycast — Google Public DNS (8.8.8.8) and Cloudflare (1.1.1.1) use anycast to serve billions of queries with low latency globally. The main challenge: if a node fails, BGP must withdraw its announcement, which can take seconds to minutes to propagate globally.
Pro Tip
Before answering, structure your response: one-line definition → real-world analogy → concrete example from a project. This makes even complex Networking answers easy to follow.
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