Without Spanning Tree Protocol (STP), frames would loop for an indefinite period of time in networks with physically redundant links. To prevent looping frames, STP blocks some ports from forwarding frames so that only one active path exists between any pair of LAN segment (collision domains).The result of STP is both good and bad. Frames do not loop infinitely, which makes the LAN usable, which is good. However, the network does not actively take advantage of some of the redundant links, because they are blocked to prevent frames from looping. Some users’ traffic travels a seemingly longer path through the network, because a shorter physical path is blocked, which is bad. However the net result (yep, I wrote that on purpose! is good. If frames looped indefinitely, the LAN would be unusable .So, STP has some minor unfortunate side effects compared to the major benefit of letting you build redundant LANs.