Without the 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 segments (collision domains). The result of STP is good: Frames do not loop infinitely, which makes the LAN usable. However, the network uses some redundant links in case of a failure, but not for balancing traffic.
To avoid loops, all bridging devices, including switches, use STP. STP causes each interface on a bridging device to settle into a blocking state or a forwarding state. Blocking means that the interface cannot forward or receive data frames.
Forwarding means that the interface can send and receive data frames.
By having a correct subset of the interfaces blocked, a single currently active logical path will exist between each pair of LANs.
STP behaves identically for a transparent bridge and a switch. So, the terms bridge, switch, and bridging device all are used interchangeably when discussing STP.
A simple example makes the need for STP more obvious. Remember, switches forward frames sent to both unknown unicast MAC addresses and the broadcast address, out all interfaces (except the incoming interface).
