Two hubs connected with two connections Port 4 Port 5 Port 6 Port 7 Port 4 Port 5 Port 6 Port 7 Hub 2 Port 0 Port 1 Port 2 Port 3 Port 0 Port 1 Port 2 Port 3 Packet is forwarded out all ports Packet is forwarded out all ports on Hub 1 on Hub 2, back to Hub 1, then back to Hub 2, etc. Port 4 Port 5 Port 6 Port 7 Port 4 Port 5 Port 6 Port 7 Hub 1 Port 0 Port 1 Port 2 Port 3 Port 0 Port 1 Port 2 Port 3 Packet enters network Endless loop Figure 2-6. Broadcast storm constantly instead of blinking normally, and (perhaps most importantly) senior execu- tives threatening you with bodily harm. The only way to resolve a broadcast storm is to break the loop. Shutting down and restarting the network devices will just start the cycle again. Because hubs are not generally manageable, it can be quite a challenge to find a layer-2 loop in a crisis. Hubs have a lot of drawbacks, and modern networks rarely employ them. Hubs have long since been replaced by switches, which offer greater speed, automatic loop detection, and a host of additional features. Switches The next step in the evolution of Ethernet after the hub was the switch. Switches dif- fer from hubs in that switches play an active role in how frames are forwarded. Remember that a hub simply repeats every signal it receives via any of its ports out every other port. A switch, in contrast, keeps track of what devices are on what ports, and forwards frames only to the devices for which they are intended. What we refer to as a packet in TCP/IP is called a frame when speak- ing about hubs, bridges, and switches. Technically, they are different things, since a TCP packet is encapsulated with layer-2 information to form a frame. However, the terms "frames" and "packets" are often thrown around interchangeably (I'm guilty of this myself). To be per- fectly correct, always refer to frames when speaking of hubs and switches. Chapter 2: Hubs and Switches When other companies began developing switches, Cisco had all of its energies con- centrated in routers, SO it did not have a so