Switches must be connected in a tree topology and not connected in such a way as to
form a ring. In other words, there must be only one path between any two devices (connected
to any of the switches). If more than one path exists between any two devices, a
loop is formed; this is unacceptable because frames can endlessly circulate over that loop,
resulting in network overload. Bridges and Switches employ Bridge Protocol Data Units
(BPDUs) to exchange information with each other regarding their individual status.
Because interconnecting multiple LANs usually means, in effect, interconnecting
hundreds of devices, identifying such loops between every combination of devices is
not done manually. The IEEE 802.1D defined an algorithm called the Spanning Tree
Protocol (STP) that will, using the appropriate BPDUs, automatically detect such loops
and disable the physical ports that enable the duplicate paths.
The STP is essentially the ???Control Plane??? of an Ethernet switch solution and
is also used to recover from failures. On detecting a failure on a path between two
devices, the STP figures out (or converges to) an alternative path and enables it for
communication.
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