What triggers IPv4 fragmentation and how does the DF flag influence it?

• A. Fragmentation is based on TTL.
• B. Fragmentation occurs for every forward; DF enables fragmentation.
• C. Fragmentation is never allowed; DF has no effect.
• D. When a packet exceeds the link MTU; DF prohibits fragmentation when set.

Answer: (D)

Fragmentation happens when a router must forward a packet that's larger than the next link's MTU. If the Don't Fragment (DF) flag isn’t set, the router can split the packet into smaller fragments so it can traverse the path and be reassembled at the destination. If the DF flag is set and fragmentation would be required, the router must drop the packet and send back an ICMP error indicating that fragmentation is needed but DF was set. This is why the DF bit directly controls whether large packets are allowed to be fragmented on a given hop.