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(Optional) If you expect multiple L2TP clients behind a NAT device to attempt L2TP over IPsec connections to the security appliance, you must enable NAT traversal so that ESP packets can pass through one or more NAT devices. Complete these steps in order to do this:

   1.

      Choose Configuration > VPN > IKE > Global Parameters.
   2.

      Ensure that ISAKMP is enabled on an interface.
   3.

      Check Enable IPSec over NAT-T.
   4.

      Click OK.

Может быть об этом речь?