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Privileged EXECDomain 2.0
show mac address-table
Displays the Layer 2 forwarding database of a Cisco switch. This table proves exactly which physical ports physical devices are plugged into, making it an essential tool for tracing cables and auditing network access.
Reading the Output
Switch# show mac address-table
Mac Address Table
-------------------------------------------
Vlan Mac Address Type Ports
---- ----------- -------- -----
All 0100.0ccc.cccc STATIC CPU
10 aaaa.bbbb.cccc DYNAMIC Fa0/1
10 1111.2222.3333 DYNAMIC Fa0/24
CCNA Exam Gotchas
[!]
Source Address Learning
A switch NEVER learns a MAC address by looking at the destination IP or destination MAC of a frame. It populates this table purely by inspecting the Source MAC Address of incoming frames hitting a port.
[!]
The 300-Second Aging Timer
DYNAMIC entries do not live forever. If a PC goes silent and sends zero frames for 300 seconds (5 minutes), the switch flushes its MAC address from the table to save RAM. If that PC wakes up and receives traffic later, the switch will be forced to flood the frame out of all ports until the PC responds and is relearned.