A host has a link light, exchanges frames with its neighbours on the local segment without trouble, and cannot reach anything on a remote network. Which layer does the model point at first?
Answer
Layer 3, because layers 1 and 2 can be entirely healthy while the address, prefix, gateway, or route is wrong
The evidence given rules the lower layers in, not out: a link light is a layer 1 pass, and successful local frame exchange is a layer 2 pass. What remains is logical addressing and forwarding between networks, which is layer 3. Signal strength governs the local link, and the local link is demonstrably working. Forwarding between networks is a router’s job, not a switch’s. And the transport layer answer inverts the order of operations — a TCP handshake requires packets to reach the far end first, so a broken layer 3 explains a failed handshake rather than the other way round.