With increased reliance on the network, many companies wanted redundancy built into their switched networks. In the event of a failure, alternative paths could still be used to switch and forward packets. One drawback of this approach is that network loops can occur. Loops form when redundant connections between switches form a circular path. This leads to a condition in which the same packets can travel endlessly around the same path in a circle.
Loops can be deadly, bringing whole networks to a halt, especially when IP services such as multicast and broadcast are enabled. Because of this possibility, it is always a good practice to break up Layer 2 networks with Layer 3 routers.