Which condition breaks Path MTU Discovery in practice?

• A. ICMP messages being blocked.
• B. DNS resolution failures.
• C. BGP neighbor adjacency flapping.
• D. ARP aging.

Answer: (A)

Path MTU Discovery relies on ICMP feedback to learn the smallest MTU along the path. When a router forwards a packet that’s too large and the Don’t Fragment bit is set, it sends back an ICMP error indicating the MTU that can be supported. The source uses that information to shrink packet sizes and avoid fragmentation on the rest of the path. If those ICMP messages are blocked or filtered, that feedback never reaches the sender, so PMTUD can’t learn the correct path MTU. Large packets then get dropped somewhere along the route, creating a silent failure known as an MTU black hole.

Other options don’t interfere with this discovery mechanism: DNS resolution failures affect hostname lookups, not MTU learning; BGP neighbor adjacency flapping affects route reachability but not the ability to receive PMTUD feedback; ARP aging deals with local address resolution on a LAN, not end-to-end path MTU discovery.