What is the PDU at Layer 2 (Data Link)?

The Layer 2 (Data Link) PDU is the frame — a packet wrapped with a header (source/destination MAC) and a trailer (FCS for error checking).

* The Layer 2 frame: MAC header, the packet, and an FCS trailer. *

The Data Link layer takes a packet from the Network layer and wraps it in both a header and a trailer to make a frame — the unit that actually crosses a single physical link. Its job is local delivery to the next device on this link, so what it adds is link-scoped addressing plus an integrity check:

• Source and destination MAC addresses (in the header) — name the sending NIC and the next NIC on this link. Unlike the IP addresses inside, these are rewritten at every hop.
• Frame Check Sequence / FCS (in the trailer) — a checksum the receiver recomputes to detect whether the frame was corrupted in transit; if it doesn't match, the frame is discarded.

The frame is the only PDU that adds a trailer as well as a header — the trailer exists specifically to carry that error-check at the end of the frame.

Memory aid: A frame "frames" the packet — header in front, trailer behind, ready for the wire.

Go deeper:

• Frame (networking) — the data-link container with sync, payload, and frame check sequence.
• Data link layer — framing, MAC addressing, and error detection.