1.1 LAN Switching
1.1.5 How a LAN switch learns addresses
An Ethernet switch can learn the address of each device on the network by:
  • reading the source address of each packet transmitted
  • noting the port where the frame was heard 

The switch then adds this information to its forwarding database. Addresses are learned dynamically. This means that as new addresses are read they are learned and stored in content addressable memory (CAM) for future use.

Each time an address is stored it is time stamped. This allows for addresses to be stored for a set period of time. Each time an address is referenced or found in the CAM, it receives a new time stamp. Addresses that are not referenced during a set period of time are removed from the list to maintain an accurate and functional forwarding database. 

Web Links
Cisco Connection Online
Cisco Documentation
Networking Tutorial Articles
Search Cisco