Anycast and GeoDNS: routing to the nearest edge

You want every user — whether in London, Tokyo, or São Paulo — to hit the nearest CDN server automatically, without manually configuring routes or maintaining separate URLs per region. Two mechanisms solve this: Anycast and GeoDNS. They work differently and are often used together.

Anycast IP routing

With Anycast, the CDN operator announces the same IP address (e.g. 93.184.216.1) from multiple geographic locations simultaneously via BGP. BGP routing selects the “nearest” edge location as measured by AS path length or hop count. When a user makes a request, their local router sends it to the closest POP advertising that IP — no special DNS needed, just standard BGP routing.

Upside: works automatically; any network-level change (POP added, withdrawn) takes effect within BGP convergence time (seconds to minutes). Downside: BGP “nearest” is hop-count nearest, not latency-nearest. Two cities equidistant by hops can differ by 50 ms of real latency.

GeoDNS

With GeoDNS, the CDN’s authoritative nameserver returns different A records based on the geographic location of the resolving nameserver. A query from a London resolver returns a London edge IP; a Tokyo resolver gets Tokyo. This gives the operator explicit control over routing — you can steer different regions to different POPs manually.

The resolver vs. the user. GeoDNS sees the resolver’s IP, not the browser’s. A user in Toronto using Google’s 8.8.8.8 resolver gets the IP for whichever region Google’s DNS server appears to come from — often Mountain View, CA — which may route them to a US POP instead of a Toronto one. That’s suboptimal.

EDNS Client Subnet (ECS) partially fixes this: resolvers that opt in send the user’s /24 subnet prefix alongside the DNS query, letting the authoritative server return a location-aware IP. Cost: the user’s subnet prefix travels up the DNS delegation chain, reducing privacy. Apple’s iCloud Private Relay and Cloudflare’s 1.1.1.1 disable ECS for this reason, accepting slightly suboptimal routing.

How CDNs combine both

Most large CDNs use Anycast at the IP layer and GeoDNS on top to combine their strengths. Anycast handles DDoS absorption well — volumetric attacks are spread across every POP advertising the IP. GeoDNS provides explicit failover: withdraw a POP from DNS when unhealthy, so new connections stop arriving within one DNS TTL.