CCNA®: Cisco® Certified Network Associate: Fast Pass, Third Edition

2.9. Describe how VLANs create logically separate networks and the need for routing between them

Figure 2.19 shows how layer 2 switched networks are typically designed—as flat networks. With this configuration, every broadcast packet transmitted is seen by every device on the network regardless of whether the device needs to receive that data or not.

By default, routers allow broadcasts to occur only within the originating network, while switches forward broadcasts to all segments. Oh, and by the way, the reason it's called a flat network is because it's one broadcast domain, not because the actual design is physically flat. In Figure 2.19 we see Host A sending out a broadcast and all ports on all switches forwarding it—all except the port that ...