Ethernet Switch MAC Address Learning

Send a Frame

Source host

Destination

Aging timeout 10 s

Ready — pick a source and destination, then send a frame.

Switch Statistics

Table entries

0 / 6

Unicast

Flooded

Forwarding Table (CAM)

MAC → HostPortAge

No entries learned yet.

How it works

A Layer-2 Ethernet switch forwards frames using MAC address learning, not routing. For every frame arriving on a port, it runs one algorithm:

on frame(srcMAC, dstMAC, inPort):
    CAM[srcMAC] = { port: inPort, t: now }      # learn
    if dstMAC in CAM and not expired(CAM[dstMAC]):
        forward(dstMAC, CAM[dstMAC].port)        # unicast
    else:
        flood(dstMAC, exclude=inPort)            # unknown unicast flood
    if now - CAM[m].t > agingTimeout: delete CAM[m]  # aging, run continuously

Learning — every frame's source MAC and the port it arrived on are recorded in the Content-Addressable Memory (CAM) table, whether or not the destination is known yet.
Flooding — if the destination MAC isn't in the table (or its entry has aged out), the switch has no choice but to send the frame out every port except the one it came in on, exactly like a hub would.
Unicast forwarding — once both hosts have exchanged at least one frame, the switch knows the exact port for each MAC and sends frames only there, which is what keeps switched networks efficient.
Aging — entries are deleted after the aging timeout (typically 300s on real hardware, shortened here) so a moved or unplugged host doesn't leave a stale port mapping; the next frame to that MAC floods again until it's relearned.

This is the exact mechanism real access switches use (Cisco, Juniper, consumer switches alike) — it's why the very first frame between two new hosts on a switched LAN is always broadcast-like, and every one after it is not.

Switch port link Unicast frame Flooded frame

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