STP and its successors, the Rapid Spanning Tree Protocol (RSTP) and the Multiple
Spanning Tree Protocol (MSTP), operate on the same principles:
?– One bridge in the network is elected root bridge and serves as the anchor for the
spanning tree.
?– One of the bridges connected to each LAN is elected designated bridge for that LAN.
This bridge is either the root bridge itself or the bridge closest to the root bridge as
measured by the costs described in the next point. The designated bridge??™s port on
a LAN is called a designated port, and always forwards data. A bridge advertises
its distance (total path cost) to the root bridge on each of its designated ports.
?– Each bridge that is not the designated bridge for a given LAN assigns a ???cost??? parameter
to that LAN, adds that cost to the cost advertised by the designated bridge,
and computes the total cost to reach the root bridge through that port. The port
with the lowest total path cost to the root bridge is that bridge??™s root port. That port
forwards data. All the other ports on that bridge that are not designated ports or
alternate ports are blocked from forwarding data.
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