正解:A,B
To prevent fragmentation in GRE tunnels, two features are configured: TCP MSS (Maximum Segment Size) and PMTUD (Path Maximum Transmission Unit Discovery). TCP MSS adjusts the maximum segment size of TCP packets to ensure they don't exceed the MTU, avoiding fragmentation. PMTUD discovers the path MTU between two IP hosts, so that IP packets can be fragmented at the source to avoid fragmentation within the network. References := Resolve IPv4 Fragmentation, MTU, MSS, and PMTUD Issues with GRE and IPsec - Cisco, GRE Tunnel MTU, Interface MTU, and Fragmentation - Cisco Community