This Counter64 counts IP datagrams the virtual router had to discard because they needed fragmentation to go out but could not be fragmented, most commonly because the Don't Fragment bit was set. A nonzero and growing value is a classic path-MTU-discovery black-hole symptom for that specific virtual router. If a customer reports intermittent large-file transfer failures through one VPN virtual router, checking this counter for that instance is a natural next step.