What is FDB?

An FDB (Forwarding Database) is a switch's MAC address table: which MAC address was last seen on which port and VLAN, used to forward frames.

A switch builds its FDB by watching traffic. Every time a frame arrives, the switch reads its source MAC address and notes "this address lives behind this port" (per VLAN on a VLAN-aware switch). When a frame needs forwarding, it looks up the destination MAC. If there is an entry, the frame goes out of that one port only. If there is no entry (an "unknown unicast"), or the destination is the broadcast address ff:ff:ff:ff:ff:ff, the switch floods the frame out of every port in the VLAN except the one it came in on. When the destination replies, its source address gets learnt too, so flooding quickly stops. Entries that see no traffic for a while are removed, so the table follows devices that move; 300 seconds is a common default ageing time. Entries can also be static, configured by hand or programmed by a control plane such as EVPN.

The term is used heavily on Linux, where bridge fdb show lists a bridge's table, and in data-centre gear; Cisco documentation tends to say "MAC address table" or "CAM table" for the same idea. Reading the FDB is a quick way to answer "which port is that device on?" A MAC address that keeps flapping between two ports usually means a loop or a misconfigured link, which is the kind of problem STP exists to prevent. The FDB is not the same as the ARP table: ARP maps IP addresses to MACs on a host or router, while the FDB maps MACs to ports on a switch. The Switching basics lesson shows a real switch learning addresses and flooding broadcasts.