OSI for CCNA Layer 2: Data Link
The CCNA exam has extensive coverage of data routing. Data routing is handled by the data link layer of the OSI. The data link layer does the following:
Transmits the data on the physical medium.
Routes the data locally on the physical network medium. The data link layer uses physical addresses assigned to each physical network device in the local network to route data from one physical device to another.
Receives each packet from the network layer on the sending host and wraps it up in a data frame along with local routing data.
Sends each data frame down to the physical layer to code an electrical or optical signal to transmit the data frame over a wire or over the air (wireless transmission).
On the receiving host, the data link layer unwraps the data frame received to extract the packet and sends it up to the network layer.
Cisco switches are Layer 2 (data link layer) devices.