Cisco is the leader in Networking hardware for Small, Medium, Large and Enterprise businesses and knowing how to properly configure a Cisco as well as know how a Cisco switch works is very crucial for any network administrator. If you have read a previous article on the OSI model you should know that a switch works at Layer 2 of the OSI model because it relies on the MAC address to determine where traffic should go.

Lets say that we have a five port switch and we plug in Computer A, Computer B and Computer C.  All switches have what is called a MAC address table where it stores that MAC address of every computer that has been connected to it. Once we plug in the three computers the MAC address table is still empty until these computers start communicating. So lets say that Computer A wants to send traffic to Computer C, so Computer A sends traffic to the switch, the switch sees that traffic is destined for Computer C but the switch does not know where Computer C is connected. Since the switch doesn’t know which port Computer C is connected to the switch will send the traffic out every switch port except the port it came in one. Now that Computer A has sent traffic to the switch the switch knows what port Computer A is connected to. So when Computer C responds to the traffic that Computer A sent the switch now knows the MAC address of Computer C and knows the MAC address of Computer A and what port is it plugged in to. So when Computer C sends traffic to Computer A the switch knows where Computer A is connected to so the traffic is sent directly to Computer A and no other computers will see the traffic.

Hope all that hasn’t confused you to much, it takes a little while to get the hang of how the flow of switches work, but once you do get it it is really easy. You can always view the MAC address table on a switch by issuing the command show mac-address and this will show you what MAC address is mapped to what port.

So you can see compared to a hub, which once traffic comes into the hub it is transmitted out every port except the one it came in on. A switch, once the MAC address table is populated traffic is directed exactly to where it needs to go and no other devices plugged into the switch will see traffic that is not directed to them.