MAC address table

The MAC address table is a construct that is maintained within the memory of switches that corresponds MAC addresses to particular interfaces and VLANs.

MAC addresses can populate the MAC address table either dynamically, by learning them from the fields contained within the Ethernet headers of the received frames, or they can be statically assigned.

An example of a MAC address table that appears with the show mac address-table command in a switch can be found below:

SW3#show mac address-table 
          Mac Address Table
-------------------------------------------

Vlan    Mac Address       Type        Ports
----    -----------       --------    -----
   1    5254.000b.225e    DYNAMIC     Gi0/0
   1    5254.0010.949f    DYNAMIC     Gi0/1
   1    5254.0010.defe    DYNAMIC     Gi0/1
   1    5254.0011.337a    DYNAMIC     Gi0/1
   1    5254.0012.4282    DYNAMIC     Gi0/1
Total Mac Addresses for this criterion: 4
SW3#

Note that the MAC addresses are assigned per port, but also indicate the VLAN to which they belong. A MAC address will remain within the MAC address table for 300 seconds by default. Every time the MAC address is detected on a port, that timer resets to 300 and begins counting down again.

Typically a MAC address should only appear once in any MAC address table. If a MAC address appears on multiple interfaces, it can result in MAC address flapping. The only exception to this rule involves the MAC address of the actual port of a switch that is configured as a trunk port.

Links

https://networklessons.com/switching/how-does-a-switch-learn-mac-addresses

https://networklessons.com/switching/static-mac-address-table-entry