Path MTU Discovery (PMTUD) is a technique used to determine the maximum transmission unit size on the network path between two IP hosts, avoiding fragmentation along the path. PMTUD works by setting the DF (Don't Fragment) bit in the packet header, which instructs routers not to fragment packets even if they exceed the maximum transmission unit of an intermediate network segment. If a packet cannot be forwarded without fragmentation, an ICMP "Packet Too Big" message should be sent back to the source host, indicating that it should reduce its packet size. However, if these ICMP messages are blocked by an access control list (ACL), PMTUD cannot function correctly because the source host will not receive feedback about necessary packet size adjustments. References: The Implementing and Operating Cisco Service Provider Network Core Technologies (SPCOR) study materials would cover PMTUD and related troubleshooting scenarios.