CTC Union FRM220-NMC, User Manual, Page: 80 / 182

Appendix A
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A.2.16.8 Spanning Tree Protocol
The Spanning Tree Protocol (STP) is a link layer (OSI Layer 2) network protocol that ensures a loop-free
topology for any bridged LAN. The basic function of STP is to prevent bridge loops and ensuing broadcast radiation.
STP is standardized as 802.1D. It creates a spanning tree within a mesh network of connected layer-2 Ethernet
switches, and disables those links that are not part of the spanning tree, leaving a single active path between any
two network nodes.
Spanning tree allows a network design to include spare (redundant) links to provide automatic backup paths if
an active link fails, without the danger of bridge loops, or the need for manual enabling/disabling of these backup
links. Bridge loops must be avoided because they result in flooding the internet network.
The following two examples that use STP are the ring and mesh topologies. Redundancy is built into these
topologies as Mesh and Ring networks are self-healing through STP.