Because broadcast and multicast frames may be of interest to all stations,
the switch or bridge normally ?¬‚oods broadcast and multicast frames to all ports except the
originating port. A switch or bridge never learns a broadcast or multicast address because
broadcast and multicast addresses never appear as the source address of a frame. This
?¬‚ooding of broadcast and multicast frames can cause a problem in a redundant switched
topology. Figure 2-17 shows how a broadcast frame from PC D would be ?¬‚ooded out all
ports on the switch.
Figure 2-17 Broadcast Flooding
Broadcast Storms
A broadcast storm occurs when each switch on a redundant network ?¬‚oods broadcast
frames endlessly. Switches ?¬‚ood broadcast frames to all ports except the port on which the
frame was received.
Example: Broadcast Storms
Figure 2-18 illustrates the problem of a broadcast storm.
The following describes the sequence of events that start a broadcast storm:
1. When host X sends a broadcast frame, such as an Address Resolution Protocol (ARP)
for its default gateway (Router Y), switch A receives the frame.
E0
A
C
0260.8c01.1111
E0: 0260.8c01.1111
E2: 0260.8c01.
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