How Spanning Tree Works


The STP algorithm creates a spanning tree of interfaces that forward frames. The tree structure creates a single path to and from each Ethernet segment, just like you can trace a single path in living, growing tree from the base of the tree to each leaf. STP actually places interfaces in forwarding state or blocking state; if an interface has no specific reason to be in forwarding state, STP places the interface in blocking state.

In other words, STP simply picks which interfaces should forward and which shouldn’t.

STP uses three criteria to choose whether to put an interface in forwarding state:

  • STP elects a root bridge. STP puts all interfaces on the root bridge in forwarding state.
  • Each nonroot bridge considers one of its ports to have the least administrative cost between itself and the root bridge. STP places this least-root-cost interface, called that bridge’s root port, in forwarding state.
  • Many bridges can attach to the same Ethernet segment. The bridge with the lowest administrative cost from itself to the root bridge, as compared with the other bridges attached to the same segment, is placed in forwarding state. The lowest- cost bridge on each segment is called the designated bridge, and that bridge’s interface, attached to that segment, is called the designated port.



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