For many organizations, the availability of the network is essential to supporting business needs. Redundancy is an important part of network design for preventing disruption of network services by minimizing the possibility of a single point of failure. One method of implementing redundancy is by installing duplicate equipment and providing failover services for critical devices.
Another method of implementing redundancy is redundant paths, as shown in the figure. Redundant paths offer alternate physical paths for data to traverse the network. Redundant paths in a switched network support high availability. However, due to the operation of switches, redundant paths in a switched Ethernet network may cause logical Layer 2 loops. For this reason, Spanning Tree Protocol (STP) is required.
STP allows for the redundancy required for reliability, but eliminates the switching loops. It does this by providing a mechanism for disabling redundant paths in a switched network until the path is necessary, such as when failures occur. STP is an open standard protocol, used in a switched environment to create a loop-free logical topology.
More details about LAN redundancy and the operation of STP are covered in the chapter titled “LAN Redundancy”.