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Adding switches to LANs can add the benefit of redundancy, that is, connecting two switches to the same network segments to ensure continual operations in case there are problems with one of the segments. Redundancy can ensure the availability of the network at all times. However, when switches are used for redundancy in a network, there is the potential problem of loops. When a host on one network segment transmits data to a host on another network segment, and the two are connected by two or more switches, each switch receives the data frames, looks up the location of the receiving device, and forwards the frame. Because each switch forwarded the frame, there is a duplication of each frame. This process results in a loop, and the frame circulates between the two paths without being removed from the network. The MAC tables might also be updated with incorrect MAC address port mapping information, resulting in inaccurate forwarding. In addition to basic connectivity problems, the proliferation of broadcast messages in networks with loops represents a serious network problem. Because of how switches operate, any multicast, broadcast, or unknown traffic will be flooded out to all ports except the incoming port. The resulting effect is a "broadcast storm" of traffic being looped endlessly through the network, almost instantly consuming the available bandwidth.

Example: Loops in a Switched Network

This looping problem is demonstrated in Figure 2-24.

Figure 2-24 Switching Loops in a Network

Segment A Segment 1

London

Segment 2

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