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When the Spanning Tree Protocol is enabled for the first time or when there is a
change in the network topology, such as a failure or the addition or removal of a
component, the Spanning Tree Protocol automatically sets up the active
topology of the current network.
Periodically, all devices running the Spanning Tree Protocol on a network
transmit packets to each other "in care of" the Bridge Group Address which all
bridges share. When a bridge receives a frame sent to the Bridge Group
Address, the bridge's Spanning Tree Protocol processes the packet. Application
software and other LAN segments ignore the packet. Bridges communicate
between each other in order to determine the Root Bridge.
During communication between bridges, one bridge is determined to have the
lowest bridge identifier. This bridge becomes the Root Bridge.
After the Root Bridge has been selected, each LAN segment looks for the
bridge that has the lowest cost relative to the Root Bridge. These bridges
become Designated Bridges.
Each Designated Bridge selects a Designated Port. This port is responsible for
forwarding packets to the Root Bridge.
When the active topology of the network is determined, all packets between any
two nodes in the network use only one path. Where a duplicate path exists, the
non-designated port is put into a blocking state.
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