CCNA Notes: Switches

A network switch is a small hardware device that joins multiple computers together within one local area network (LAN). Technically, network switches operate at layer two (Data Link Layer) of the OSI model.

Network switches appear nearly identical to network hubs, but a switch generally contains more intelligence (and a slightly higher price tag) than a hub. Unlike hubs, network switches are capable of inspecting data packets as they are received, determining the source and destination device of each packet, and forwarding them appropriately. By delivering messages only to the connected device intended, a network switch conserves network bandwidth and offers generally better performance than a hub.

As with hubs, Ethernet implementations of network switches are the most common. Mainstream Ethernet network switches support either 10/100 Mbps Fast Ethernet or Gigabit Ethernet (10/100/1000) standards.

MICROSEGMENTATION = one host one collision domain

First Decision:

A cisco switch is gonna do 1 of 3 things to an incoming frame…..

Forward

Flood

Filter

The switch will look into its mac-address table to make this decision. First The switch is gonna check the source Mac-address to make the mac-address table.

Lets suppose we put a new switch on a network and it doesn’t know any mac address.

Host A wants to send frame to Host C, when the frame reaches switch it has to make one of the above decisions but first the switch will make an entry for the source mac-address as it is newly added and doesn’t know any mac.

Now it ll look for dest. Mac and it does not have an entry for dest. Mac so the switch will Flood the frame to all ports except the one it came in on. This is called unknown unicast frame . And flood is always a broadcast.

Now host C will reply to Host A

The switch ll look into its mac address table to check the entry for source mac which is all c's address, it is not going to find it so it'll add it.

Now the switch will check dest. Mac address and it is on the mac-address table. So it will FORWARD the frame to host A.

Now lets assume that the switch has learnt all the 4 mac-addresses .

Now the Host A want to send a Frame to host B.

When the frame reaches switch , switch will look into its mac address table for source, It is gonna find it, then it ll look for dest. Mac address and it has both in its table but they are both on the same port, so switch is gonna DROP (FILTER) the frame.

SWITCHES NEVER SEND A FRAME BACK TO THE PORT IT CAME IN ON…