Anycast vs Unicast

Different routing.

Overview

Anycast and unicast are two IP-routing strategies that solve different problems. Anycast announces the same IP from multiple sites and lets BGP route each client to the topologically nearest one; unicast routes to one specific origin. The right answer depends on whether you need global low latency and DDoS absorption (anycast) or predictable single-origin debugging (unicast).

• Anycast: nearest origin via BGP. Same IP announced from multiple sites; BGP picks the topologically nearest. Low latency globally, automatic regional failover.
• Unicast: one IP, one origin. Standard internet routing. Simple, predictable, easy to debug because the path is deterministic.
• Anycast plus DDoS resilience. Attack traffic spreads across announcing sites. The blast radius of any single attack is bounded.
• Anycast plus automatic failover. Site failure removes its BGP announcement; traffic shifts to the next-nearest site without DNS changes.

The approach

Unicast by default, anycast for globally distributed services where it earns its operational complexity. DNS, CDN, and DDoS-protection layers are the canonical anycast use cases; application origins typically stay on unicast.

• Unicast for application origins. Most application traffic. Simple ops, deterministic debugging.
• Anycast for authoritative DNS. Global low latency on the resolver path. Standard pattern for production DNS.
• Anycast for CDN edges. Edges announce the same IP from every POP. Standard CDN architecture.
• Anycast for DDoS protection plus documented choice. Attack absorption across sites; per-property rationale captured for the audit trail.

Why this compounds

The right routing choice compounds across years. Each anycast property unlocks global latency reductions and DDoS resilience that unicast cannot match. Each unicast property keeps debugging simple where global latency does not matter. Team expertise in BGP grows with each deliberate choice.

• Lower global latency. Anycast for the right workloads. Users see sub-50ms responses globally.
• Better resilience. Anycast survives site failures automatically. Uptime improves at the routing layer.
• Predictable debugging. Unicast where appropriate keeps investigation simple. Path is deterministic.
• Year-one investment, year-two habit. First year builds BGP fluency; by year two, the choice is automatic per workload.