When NAT traversal is enabled on the SonicWALL, it may not be possible to pass traffic over a VPN tunnel after it has been successfully established, despite correct configuration of the appliances on both ends of the connection. Check the following troubleshooting areas to resolve the issue:
Disable NAT traversal on the VPN > Advanced tab of the SonicWALL. This feature is not always needed.
Disable IKE Dead Peer Detection and set the Keep Alive interval to 0 on the SonicWALL.
Make sure any upstream firewall or router is configured to allow UDP 500 and UDP 4500 traffic to the WAN addresses of both the SonicWALL and the 3rd party product.
Make sure both the SonicWALL and 3rd party appliance are running the latest available firmware versions to take advantage of full NAT-T compatibility as certified by ICSA and VPNC.
See the following resources for additional details on certification,NAT traversal and compatibility: