Chapter 11 IPsec VPN for FortiOS 5.0 : Monitoring and troubleshooting : VPN troubleshooting tips : Check Phase 1 proposal settings
  
Check Phase 1 proposal settings
Ensure that both sides have at least one Phase 1 proposal in common. Otherwise they will not connect. If there are many proposals in the list, this will slow down the negotiating of Phase 1. If its too slow, the connection may timeout before completing. If this happens, try removing some of the unused proposals.
NPU offloading is supported when the local gateway is a loopback interface.