Introduction When migrating from on-premise to cloud, maintaining hybrid network connectivity is critical for applications that still depend on on-premise resources. Lost connectivity breaks these dependencies.

Symptoms - On-premise servers cannot reach cloud resources - VPN tunnel down after migration - Error: "Connection timed out" for cross-network communication - Cloud instances cannot reach on-premise databases - DNS resolution failing between environments

Common Causes - VPN configuration not updated after cloud network change - Route tables not propagating routes between environments - Security group blocking cross-network traffic - NAT gateway not configured for return traffic - Overlapping IP addresses between environments

Step-by-Step Fix 1. **Test connectivity between environments': ```bash # From cloud to on-prem ping <on-prem-ip> traceroute <on-prem-ip> # From on-prem to cloud ping <cloud-private-ip> ```

  1. 1.**Check VPN/Direct Connect status':
  2. 2.```bash
  3. 3.# AWS VPN
  4. 4.aws ec2 describe-vpn-connections --filters Name=state,Values=available
  5. 5.# Check VPN tunnel status
  6. 6.`
  7. 7.**Verify route tables':
  8. 8.```bash
  9. 9.# Cloud side
  10. 10.aws ec2 describe-route-tables --filters Name=vpc-id,Values=<vpc-id>
  11. 11.# On-premise side
  12. 12.ip route show
  13. 13.`

Prevention - Test hybrid connectivity before migration - Document all cross-network dependencies - Use non-overlapping IP address ranges - Implement connectivity monitoring between environments - Have fallback connectivity (secondary VPN) during migration