A remote office router uses a GRE tunnel over an MPLS link. TCP sessions across the tunnel show frequent path MTU issues and fragmentation. Which single action will most directly resolve the problem without changing the physical MTU on the WAN interface?

The correct solution is to enable ip tcp adjust-mss on the router's inside interface. This command reduces the TCP Maximum Segment Size (MSS) advertised during the TCP three-way handshake. By lowering the MSS, the router ensures that TCP segments are smaller, preventing them from exceeding the effective MTU of the GRE tunnel, which adds overhead (24 bytes for GRE and 20 bytes for the outer IP header) to the encapsulated packets. This avoids fragmentation and the associated performance degradation. Increasing the GRE tunnel's IP MTU above the physical interface MTU would cause packets to be fragmented by the physical interface, exacerbating the problem. Disabling GRE checksum reduces a small amount of overhead but does not address the fundamental MTU mismatch causing fragmentation. Applying a crypto map is for encryption and does not directly resolve MTU issues; fragmentation is generally undesirable for performance.