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Bridging, Switching, and Spanning Tree 151
Figure 4-9
Collision Domains
NOTE
Many vendors, including Cisco, sell cards in switches that do not switch on all ports. In other
words, the equivalent of a shared hub with several ports is built into a card rather than each port
being treated as its own collision domain. Frames destined for a MAC address off one of these
ports are sent out all these ports by the switch. The switch ports in the figures in this chapter are
all switched, unless otherwise specified.
The broadcast domain concept is similar to the concept of collision domains; however, only
routers stop the flow of broadcasts. Figure 4-10 provides the broadcast domains for the same
network depicted in Figure 4-9.
The broadcast domain is not affected by the inclusion or exclusion of switches or bridges. The
router creates its own broadcasts (RIP, IGRP, SAP, and so on), but the router does not forward
broadcasts received in the left-side interface out the right-side interface. In other words,
broadcasts created and sent by a device in one broadcast domain are not sent to devices in
another broadcast domain.
ch04.fm Page 151 Monday, March 20, 2000 5:02 PM