Frame Layer 2 Protocol Data Unit (PDU)
The fundamental unit of data transmission on a local area network (LAN). When a Layer 3 router generates an IP Packet, it must "package" that packet into a Layer 2 Frame before it can be physically transmitted across an Ethernet cable or Wi-Fi frequency. Frames only exist locally; routers destroy them and build brand new ones at every hop.
Anatomy of an Ethernet II Frame
| Header Field | Purpose & Size |
|---|---|
| Preamble / SFD | 8 Bytes: An alternating pattern of 1s and 0s used to wake up receiving network cards and synchronize their hardware clocks. The final byte is the Start Frame Delimiter (SFD), signaling the payload is about to begin. |
| Destination MAC | 6 Bytes: The physical address of the device intended to receive the frame. Switches read this specific field to make forwarding decisions. |
| Source MAC | 6 Bytes: The physical address of the device that sent the frame. Switches read this specific field to populate their MAC Address Tables. |
| EtherType | 2 Bytes: Identifies the Layer 3 protocol sitting inside the payload. (e.g., 0x0800 means an IPv4 packet is inside, 0x86DD means IPv6). |
| Payload (Data) | 46 to 1500 Bytes: The actual data being transported (the encapsulated Layer 3 packet). If the payload is smaller than 46 bytes, "padding" is added to meet the minimum size requirement. |
| FCS (Trailer) | 4 Bytes: The Frame Check Sequence. The sender runs a mathematical hash (CRC) against the frame and stores the result here. The receiver runs the exact same hash; if the numbers don't match, the frame was corrupted in transit. |
CCNA Exam Gotchas
Silent Dropping (No Recovery)
If the FCS hash fails because a frame was corrupted by electrical interference on the wire, the receiving switch immediately destroys the frame. Layer 2 does not possess a recovery or request-retry mechanism. It drops the frame silently. It is entirely up to upper-layer protocols (like TCP at Layer 4) to notice the missing data and request a retransmission.
Runts and Giants
The absolute standard boundaries for an Ethernet frame are a minimum of 64 bytes and a maximum of 1518 bytes (including headers, but excluding the preamble). Frames smaller than 64 bytes are called Runts (usually caused by collisions). Frames larger than 1518 bytes are called Giants. A standard switch will drop both as invalid unless explicitly configured to accept Jumbo Frames.