To resolve the problem, first, try to reset the Azure VPN gateway and reset the tunnel from the on-premises VPN device. If the problem persists, follow these steps to identify the cause of the problem.
Step 1. Check whether the on-premises VPN device is validated
Step 2. Verify the shared key
Step 3. Verify the VPN peer IPs
Step 4. Check UDR and NSGs on the gateway subnet
Step 5. Check the on-premises VPN device external interface address
Step 6. Verify that the subnets match exactly (Azure policy-based gateways)
Step 7. Verify the Azure gateway health probe
Step 8. Check whether the on-premises VPN device has the perfect forward secrecy feature enabled
Hope this could help you.
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