When designing a switched or bridged network, you’ll almost certainly need to consider redundancy. While network redundancy is a great idea in principle, there are issues that you’ll need to be aware of. The biggest issue is that bridging redundancy exposes networks to a loop, and loops cause major problems if not dealt with properly.
The problems associated with network loops go back to the days when bridging was the primary way of segmenting a LAN. The idea was to have more than one bridge connecting two segments, in order to provide a redundant path should a link or bridge fail. The problem with having this redundancy in a bridged environment is that may create loops, and network loops are capable of causing communication problems. In the case of a bridging loop, a network becomes susceptible to broadcast storms.
In this section you’ll notice that I tend to refer to bridges instead of switches. The main reason is that a bridged network is easier to diagram that one using switches. For all intents and purposes, when describing loops the terms bridge and switch can be used interchangeably.