Self Learning Properties of Link Layer Switches
A switch has the wonderful property that its table built automatically, dynamically, and autonomously, without any intervention from a network administrator or from a configuration protocol. In other words, switches are self-learning.
Fig: An institutional network connected together by four switches
This capability accomplished as follows:
- The switch table is initially empty.
- For each incoming frame received on an interface, the switch stores in its table
- The MAC address in the frame’s source address field,
- The interface from which the frame arrived
- The current time.
- In this manner, the switch records in its table the LAN segment on which the sender resides. If every host in the LAN eventually sends a frame, then every host will eventually get recorded in the table.
The switch deletes an address in the table if no frames received with that address as the source address after some period of time (the aging time).
In this manner, if a PC replaced by another PC (with a different adapter), the MAC address of the original PC will eventually purge from the switch table.
Let’s walk through the self-learning property for the uppermost switch in Figure 15.
Moreover, Suppose at time 9:39 a frame with source address 01-12-23-34-45-56 arrives from the interface
Also, Suppose that this address is not on the switch table. Then the switch adds a new entry to the table.
Continuing with this same example, suppose that the aging time for this switch is 60 minutes, and no frames with source address 62-FE-F7-11-89-A3 arrive at the switch between 9:32 and 10:32. Then at time 10:32, the switch removes this address from its table.