Allied Telesis AT-S21 - Manual (Page 81)

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This appendix provides a brief explanation of the Spanning Tree
Algorithm as implemented by Allied Telesyn on the switches.
For detailed information on the operation of the Spanning Tree
Algorithm, consult Section 4 of IEEE Std 802.1D, ISO/IEC 10038: 1993.
The AT-8118, AT-8118XL, AT-8124XL, and AT-8126XL switches, which
run the AT-S21 software, implement the IEEE 802.1D Spanning Tree
Protocol (STP). The STP provides a network with robustness and
allows network administrators to easily change their network
topology. Its implementation reduces complex network topologies
(networks with multiple paths between source and destination
nodes) to a single active topology. This technique guarantees that
loops do not occur between source and destination nodes of the
network. Loops are eliminated by placing some of the redundant
ports in a “blocking” state, in which they do not forward packets but
continue to execute the protocol. If the network topology changes,
for example by the failure, removal, or addition of an active network
node, a “blocked” port may be included in the new active topology
and begin forwarding frames.