You have configured a Site-to-Site IPsec VPN tunnel between a FortiGate and a third-party device but notice that one of the error counters on the tunnel interface keeps increasing. Which two configuration options can resolve this problem? (Choose two.)

Select an option, then click Submit answer.
Reference / correct answer:
Adjust the MTU of the IPsec interface.
Most accepted answer: D. Adjust the MTU of the IPsec interface.
Community votes: A=1, B=1, C=1, D=3
Selected Answer: CD The growing error counter is typical of MTU/fragmentation issues on IPsec tunnels. Fix by tuning MTU: C – Lower/adjust the MTU on the physical interface carrying the IPsec traffic. D – Lower/adjust the MTU on the IPsec interface itself (enable MTU override and set an appropriate value). (Forward Error Correction and DF-bit honoring won’t address this counter directly.) C and D directly fix the MTU mismatch by lowering the packet size on the physical and IPsec interfaces to account for encryption overhead, eliminating the cause of TX errors. B (DF-bit honoring) only helps if Path MTU Discovery works end-to-end, which often fails due to blocked ICMP — so it’s less reliable. upvoted 2 times
Selected Answer: BD A can't be correct as the tunnel is to a third party device upvoted 1 times
Selected Answer: AD Not sure on the missing answer. A: FEC can help reconstruct the lost data. B: the DF bit enabled would make it so if the MTU of the packets were bigger than the physical interface MTU then it would drop, so this is better to be disabled. C: Adjusting the MTU of the physical interface effects all traffic through that interface. D: Adjusting the MTU of the IPsec would allow the encrypted payload to fit into the size of the interfaces MTU, preventing the drop of packets. upvoted 1 times
original question is with choose three option + missin here an additional answer upvoted 1 times