Layer 2 Switch – MAC Address Table and Frame Forwarding with Hub

A Layer 2 switch learns most of its information about the location of other endpoints via "listening" to ingressing frames, and when it is not aware of the location, it uses floodingand will learn from the answer. Lets say the topology is:

(Host A) <--> (Switch A) <--> (Switch B) <-->(Host B).

Also important to note, a L2 Switch forwards, it does not route. By that I mean, when it receives an encapsulated frame, it forwards it out interfaces.

• Host A wants to get to Host B. Host A knows Host B's MAC address. So Host A sends a frame out to Switch A.
• Switch A does not have Host B in its MAC address Table. Switch A then flood out the frame on all of its ports, except for the one Host A is located. Among all those ports there is the ports that Switch B is plugged into, so Switch B receives the frame.
• Switch B then flood that frame out all of its ports (considering Switch B does not have Host B in its MAC address Table).
• Host B will respond to the frame, Switch B will then record Host B MAC address and send the response to Switch A. Switch A then records the MAC address of Host B in its table and send the response to Host A

Here's a related question: How does a switch learn a switch table?

A typical timeout is 5 min (often called mac-aging). Re-learning MAC addresses can be dangerous in terms of security. On the other hand, when you disconnect your computer from the switch, all MACs associated with the port to which it was connected are flushed immediately; when you reconnect your computer (to the same or a different port), the switch can relearn the appropriate mapping.

Please be aware that, while this is typical behaviour, there are some switches configured to stick with a MAC once it was learnt (or to limit the number of MACs that can be dynamically learnt: once the limit is reached, manual intervention is required).