Question 1150
At Oakmont Regional Hospital, the FortiGate 7.6 security administration team encounters the following authorized operational case: a file transfer fails only for larger payloads across a healthy IPsec tunnel. They need a defensible FortiOS configuration, diagnosis, validation or prevention decision. What root condition most directly explains the described FortiGate symptom?
Explanation
Correct Answer
ESP and optional NAT-T add packet overhead that can exceed the underlay path payload size. The mechanism is relevant because the stated ipsec vpn mtu and fragmentation incident concerns the actual selected path, policy, identity or inspection state. The appropriate response is to adjust the supported path MTU or TCP MSS behavior after measuring encapsulation overhead and PMTUD; verify it through this observed condition: a supervised large transfer succeeds without recurring fragmentation-related loss.
Incorrect Answers
Two logical tunnel interfaces sharing one upstream circuit do not protect against that circuit failure explains a different situation, redundant vpn split provider design, where two site-to-site IPsec tunnels terminate on the same failing ISP despite being labeled primary and backup. It does not account for the specific observed ipsec vpn mtu and fragmentation condition at Oakmont Regional Hospital. The decisive mechanism here is: ESP and optional NAT-T add packet overhead that can exceed the underlay path payload size.
One tunnel on one underlay remains a single point of failure without a separate operational path explains a different situation, ipsec redundant secondary tunnel, where a branch VPN uses one tunnel with no alternate path and every carrier outage disconnects the remote office. It does not account for the specific observed ipsec vpn mtu and fragmentation condition at Oakmont Regional Hospital. The decisive mechanism here is: ESP and optional NAT-T add packet overhead that can exceed the underlay path payload size.
An accepting outbound policy does not necessarily create permission for independently initiated inbound flows explains a different situation, ipsec tunnel policy return direction, where the head office reaches a remote file server but the remote office cannot initiate sessions toward the head-office database. It does not account for the specific observed ipsec vpn mtu and fragmentation condition at Oakmont Regional Hospital. The decisive mechanism here is: ESP and optional NAT-T add packet overhead that can exceed the underlay path payload size.