Site-to-Site VPN Connection Status Changes to Unknown

Question

Hello,

I set up a Site-to-Site VPN connection from an Azure VNET to an on-premise network to access a private SMS gateway, following this tutorial: Site-to-Site VPN Gateway Setup.

Here’s the setup:

• The on-premise VPN device is supported.
• The local network gateway is configured with the VPN device's public IP and two address ranges: sms-gateway-private-ip-1/32 and sms-gateway-private-ip-2/32.

The connection was working fine with a status of Connected. However, it suddenly changed to Unknown.

Troubleshooting details:

1. Connection stats show:
   • Ingress Packets Dropped due to Traffic Selector Mismatch: 0 Packets
     • Egress Packets Dropped due to Traffic Selector Mismatch: 0 Packets
     1. VPN Gateway Resource Health reports:

     At 05:40 PM, Wednesday, 20 November 2024 UTC, the Azure monitoring system received the following information regarding your VPN connection: The connection cannot be established because the other VPN device is unreachable. If the on-premises VPN device is unreachable or not responding to the Azure VPN gateway IKE handshake, the VPN connection cannot establish.

When we restarted the IPSec tunnel on the on-premise VPN device, the connection briefly showed Connected but reverted to Unknown shortly afterward.

Question:

• What could be causing this issue, and how can I resolve it?
• Are there additional diagnostics or configurations I should check on either the Azure side or the on-premise VPN device?

Any guidance would be greatly appreciated!