What Is a MAC Address Table?
a MAC Address Table — the switch's memory of which MAC address lives on which port — the lookup table it uses to forward frames intelligently instead of flooding everything.
How it works
The switch learns by watching source MACs: a frame arriving on port 5 from MAC A teaches it "A lives on port 5". Future frames destined to A go only to port 5. Unknown destinations are flooded to all ports until learned. Entries age out (default 300s) if a device goes quiet.
Why it matters
The MAC table (CAM table) is how switching actually works — and reading it (show mac address-table) is how you physically locate any device on the network. MAC flapping in this table is the classic signature of a Layer 2 loop.
Frequently asked questions
How does a switch build its MAC address table?
By recording the source MAC address and arrival port of every frame it receives — learning where each device lives.
What happens when the destination MAC isn't in the table?
The switch floods the frame out all ports in that VLAN (except the arrival port) until the destination replies and gets learned.
What is MAC flapping?
The same MAC address rapidly appearing on different ports — usually indicating a Layer 2 loop, and a signal to check spanning tree immediately.
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