Spanning Tree Protocol (IEEE 802.1D)
A LAN with redundant links would cause Ethernet frames to loop for an indefinite period of time unless some other mechanism stops the frames from looping. The fundamental frame-forwarding logic in a switch does not prevent such loops.
Historically, Spanning Tree Protocol (STP) was the first loop-prevention mechanism for Ethernet networks that used bridges and later switches. With STP enabled, some switches block ports so that these ports do not forward frames. In effect, the LAN can have redundant links for backup purposes, but STP logically stops using some links to remove the loops from the network.
STP intelligently chooses which ports to block, with two goals in mind:
All devices in a VLAN can send frames ...
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