PMTU Setting for IPSec
This advanced interface setting applies to external interfaces only. You configure this in the Advanced tab when you edit an external interface.
The PMTU settings in Fireware Web UI
The PMTU settings in Policy Manager
The Path Maximum Transmission Unit (PMTU) setting controls the length of time that the Firebox lowers the MTU for an IPSec VPN tunnel when it gets an ICMP Request to Fragment packet from a router with a lower MTU setting on the Internet.
We recommend that you keep the default setting. This can protect you from a router on the Internet with a very low MTU setting.