Trace the traffic with packet captures

Using packet captures to determine where the traffic is or isn’t flowing is one of the most helpful troubleshooting techniques. Start with the internal interface (commonly LAN) on the side where the traffic is being initiated, progress to the tun interface on that firewall, then the tun interface on the remote firewall, and finally the inside interface on the remote firewall. Determining where the traffic is seen and where it isn’t can help greatly in narrowing down where the problem is located.