 a central hub. When a frame enters the hub on the bottom left on Port 1, the frame is repeated out every other port on that hub, which includes a connection to the central hub. The central hub in turn repeats the frame out every port, propagating it to the remaining hubs in the network. This design replicates the backbone idea, in that every device on the network will receive every frame sent on the network. Port 4 Port 5 Port 6 Port 7 Port 4 Port 5 Port 6 Port 7 Port 0 Port 1 Port 2 Port 3 Port 0 Port 1 Port 2 Port 3 Port 4 Port 5 Port 6 Port 7 Port 0 Port 1 Port 2 Port 3 Port 4 Port 5 Port 6 Port 7 Port 4 Port 5 Port 6 Port 7 Port 0 Port 1 Port 2 Port 3 Port 0 Port 1 Port 2 Port 3 Figure 2-5. Hub-based network Chapter 2: Hubs and Switches In large networks of this type, new problems can arise. Late collisions occur when two stations successfully test for a clear network and then transmit, only to encounter a collision. This condition can occur when the network is SO large that the propagation of a transmitted frame from one end of the network to the other takes longer than the test used to detect whether the network is clear. One of the other major problems when using hubs is the possibility of broadcast storms. Figure 2-6 shows two hubs connected with two connections. A frame enters the network on Hub 1 and is replicated on every port, which includes the two connec- tions to Hub 2, which now repeats the frame out all of its ports, including the two ports connecting the two switches. Once Hub 1 receives the frame, it again repeats it out every interface, effectively causing an endless loop. 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 En