Atlas Network Engineer • • 5 min Global Traffic and DNS Routing Architecture with BGP Anycast
How announcing a single IP prefix from multiple global PoPs routes clients to the closest data center.
What is Anycast and How Does It Differ from Unicast?
In traditional Unicast networking, every IP address maps globally to a single server interface. Whether a client sends packets from Istanbul or Tokyo, traffic travels across physical continents to the same physical node.
With BGP Anycast, the exact same IP prefix (typically a /24 IPv4 block) is announced simultaneously by multiple edge data centers across the globe to upstream transit providers via BGP (Border Gateway Protocol) sessions.
[ Client (Europe) ] ------> [ PoP Frankfurt (198.51.100.1) ]
[ Client (Asia) ] ------> [ PoP Singapore (198.51.100.1) ]
[ Client (America)] ------> [ PoP Virginia (198.51.100.1) ]
BGP AS-Path and Routing Decisions
Internet border routers determine the best path towards the Anycast IP based on the shortest AS-Path length and local BGP metrics:
- Latency (RTT) Reduction: User requests terminate at the geographically closest edge PoP rather than crossing intercontinental backbones.
- Distributed DDoS Mitigation: Volumetric attacks are distributed and absorbed at local PoPs nearest to the attack origins, preventing global outages.
The State Challenge: TCP Flapping
If upstream routing flaps, stateful TCP sessions can shift between PoPs, breaking active connections. For this reason, Anycast is best suited for DNS (UDP 53), NTP (UDP 123), and resilient stateless reverse proxies with fast session resumption.