Troubleshooting Tip: How to change the MTU to fix an ESP fragmentation Issue

[Dongfang_Li_FTNT]

Description	This article describes how to fix an ESP fragmentation issue by changing the METU size.
Scope	All supported versions of FortiOS.
Solution	When traffic is sent to the IPSec tunnel from the local FortiGate and it is not received by the remote FortiGate, it is possible to run sniffer in the remote FortiGate to check the ESP packet to see if there is an error or drop in the ESP packet. diag sniffer packet any 'host 192.168.10.10 and proto 50' 4 Where 192.168.10.10 is the FortiGate initiates traffic.   If there is ESP fragmentation, for example: Original direction traffic has fragmentation, but reply traffic is fine. The user can reduce the MTU in the IPsec VPN tunnel interface in the source FortiGate 192.168.10.10: config system interface edit <tunnel interface> set mtu-override enable set mtu <integer> next end