IPSec VPN Tunnels suddenly failed to work, flushing tunnels required
IPSec VPN Tunnels suddenly failed to work, even the Firewall(s) show the Site-to-Site VPN working.
Required us to restart the VPN(s).
show vpn ipsec phase1-interface
How to
// FortiOS 7.2
1. To disable the tunnel
diagnose vpn tunnel down <tunnel-name>
2. Wait for a few seconds
3.To enable back the tunnel
diagnose vpn tunnel up <tunnel-name>
Might also need to (our case didn't need it):
diagnose vpn ipsec phase2 down <tunnel-name>
diagnose vpn ipsec phase2 up <tunnel-name>
Flushing the tunnel on the another Firewall also required
// FortiOS 6.2
diagnose vpn ike gateway clear name <tunnel-name>
References
Technical Tip: How to flush a VPN tunnel
Troubleshooting Tip: IPsec VPNs tunnels