BROCADE IP PRIMER

Layer 1 Ethernet (Cabling)SC connectorLC connectormini - GBICHubWith twisted pair, you need to have some way to have more than two computersshare information with each other. In the beginning, this device was calleda hub.A hub is a device with many RJ45 ports on it. As a device, it's not remarkablyintelligent. It has a very straight-forward purpose. Whenever it receives an electricalsignal on one of its ports, it replicates that signal out all of its other ports.That's it.This method is similar, in some respects, to the bus topology we talked aboutearlier. All of the computers connected to the hub (or the main line, in bustopology) saw all of the frames that were sent. It didn't matter whether theframes were meant for that computer or not, it saw them. Now, by default, theEthernet protocol ignores the frames not specifically addressed for its ownNIC. Still, though, you can see that this is a fairly inefficient way to communicate.It's also potentially insecure.SwitchWhile hubs still exist, you will find that, more commonly, they have beenreplaced by the switch. The switch looks very similar to a hub. It's a device withseveral RJ45 ports on it. What sets it apart is that it has a brain (a CPU). Wherea hub is strictly a Physical Layer device (it just replicates the signal), a switch isa Data Link Layer device.When the switch receives a signal on one of its ports, it keeps track of physicaladdresses connected to it. It then has an address book to draw from. When asignal comes in, the switch checks to see what physical address the frame isbound for. It will then look at its address book to see if it knows what physicalport (of itself) the destination physical address is connected to. If the switchfinds the destination address, it will forward the frame out of that port and onlythat port. If the switch doesn't find the destination address in its address book,it will act like a hub, and forward the frame out all of its ports. The difference isthat the switch will pay attention to replies. If the switch gets a response framefrom the destination address, it will add that address to its address book andremember it for next time.Brocade IP Primer 27