The spanning tree network protocol provides a loop free topology for any bridged LAN. The Spanning Tree Protocol, which is also referred to as STP, is defined in the IEEE Standard 802.1D. Spanning tree is based on an algorithm invented by Radia Perlman while working for Digital Equipment Corporation. STP is used in switched networks to prevent loops, and has been standardized by IEEE 802.1D. As the name suggests it finds a spanning tree within the mesh network formed by the installer, whether purposefully created or not, and disables the links not part of that tree. There are 2 different standards of STP; IEEE and DEC. IEEE is the most common, and is recommended to be the one used in a network. Problems can be caused in networks where both standards of STP are implemented. It is likely that both standards will elect their own root bridge and cause loops in the topology. The way in which both standards handle Bridge Protocol Data Units (see below) also differs. If more than one open path were to be active at once then there would be several problems. First, a broadcast storm caused by broadcast packets looping between switches would reduce available CPU resources and bandwidth (many modern bridges can detect and limit the effects of this, although it usually still causes a breakdown in connectivity). Second, the traditional source-based location system (filtering database) used by switches would fail to operate correctly. However, a good network design should include spare (redundant) links to provide an alternate path if one fails. Hence the need for spanning tree in a switched network.