This read-only Counter reports the number of times this node sent an ICMP destination-unreachable message because a datagram needed fragmentation but had the Don't Fragment flag set. An admin diagnosing an MTU mismatch affecting traffic passing through this node would watch this counter as it reflects this node informing senders that their packets are too large for the outgoing path. For example, a rising cnticmpOutDuFrags count would suggest an upstream client needs to lower its MTU or enable path MTU discovery.