enabled : If an IP packet could not be delivered/forwarded to destination due to MTU/Dont-Fragment error (packet must be fragmented due to interface-MTU but Dont-Fragment (DF) bit is set in IP header), the IP packet is discarded and the system sends an ICMP 'Destination-Unreachable/Fragment' message back to the originator of the packet. This is the default behavior of the system. disabled: If an IP packet could not be delivered/forwarded to destination due to interface-MTU/DF-bit problem, the packet is silently discarded. ICMP 'Destination-UnreachableFragment' messages should be disabled with care (only if really necessary). Disabling of this ICMP messages will make Path MTU Discovery impossible and might lead to bad performance behaviours.