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Problem: 
Troubleshooting from the FortiGate, when the traffic stops flowing, I see that
the tunnels are both up, but no data flow. Also noticed in Diagnostic logs that
we are sending traffic out of the WAN but nothing coming back.
 
Solution:
Switching the SA to the initiator only on the VPN Gateway seems to have mitigated the Secure Association issue for the VPN tunnel Quick mode.
 
Case: Case Link
Keywords: Virtual Private Network, Networking, Azure