An L2 switch makes forwarding decisions by destination MAC. It builds its table from source MACs of arriving frames. This chapter walks through MAC learning, flooding, and the difference between a hub, switch, and router.
| Field | Bytes | Purpose |
|---|---|---|
| Preamble + SFD | 8 | Sync + start-of-frame |
| Destination MAC | 6 | Who is supposed to receive it |
| Source MAC | 6 | Who sent it |
| Type/Length | 2 | Higher-layer protocol (0x0800 = IPv4, 0x86DD = IPv6, 0x0806 = ARP) |
| Payload | 46–1500 | The IP packet (or other L3 PDU) |
| FCS | 4 | CRC32 integrity check |
When a frame arrives on port X, the switch records 'source MAC is reachable through port X' in its MAC address table. This is dynamic — entries age out after a few minutes (default 300 seconds on most switches) of inactivity.
When the switch needs to forward a frame, it looks up the destination MAC. If found, the frame is sent only out the matching port (forward). If not found, the frame is sent out every port except the ingress port (flood). If the destination MAC is the broadcast FF:FF:FF:FF:FF:FF, the frame is always flooded. If the matching port is the same as the ingress port, the frame is dropped (loop avoidance).
2 more sections in this chapter, plus a 2-question quiz.
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