Network routing during a Disaster

You have your servers ready in Region B. But your users are still trying to hit Region A. How do you move them?

The DNS Swing (The "Hard" Way)

You update your DNS A-record to point to the new Load Balancer IP.

The Trap: TTL (Time to Live). If your TTL is 300 seconds (5 mins), users will continue to hit the dead region for 5 minutes. Some ISPs ignore TTLs and cache for longer. This is known as "DNS Propagation delay."

The Health Check Swing (The "Smart" Way)

Use a DNS service like Route 53 with Health Checks. The DNS service monitors your endpoint every 10 seconds. If it fails 3 times, it automatically stops returning that IP and starts returning the DR IP. This is faster and requires no human intervention.

The BGP Swing (The "Pro" Way)

If you own your own IP block (BYOIP), you can advertise it from Region A and Region B. If Region A goes dark, the BGP advertisement stops. The global internet routing table updates, and traffic naturally flows to Region B. This is how Google and Facebook stay online.