What is the primary reason packets with a large size are not arriving when sent through a GRE tunnel?

When dealing with GRE (Generic Routing Encapsulation) tunnels, one critical aspect that affects packet transmission is the Maximum Transmission Unit (MTU) size. The correct answer focuses on the fact that the don't-fragment (DF) bit is set on the traffic.

In scenarios where the DF bit is set, routers along the path are instructed not to fragment the packets, meaning that if a packet exceeds the MTU size of the GRE tunnel or any link along the path, the packet will be dropped instead of being fragmented. This leads to packets being lost, particularly larger packets that cannot be accommodated within the imposed size limitations. GRE tunnels typically encapsulate packets, adding additional headers to the original packets, which effectively reduces the MTU available for the original packet payload. Hence, if the original packets are larger than this adjusted size and the DF bit is set, they will not successfully traverse the tunnel.

Recognizing the implications of the DF bit is crucial for troubleshooting issues related to packet delivery in GRE tunnels, particularly highlighting the importance of managing MTU sizes for successful communication.