This topic explains the roles assigned to ports by the Spanning Tree algorithm and how these roles affect network topology and redundancy.
The Spanning Tree algorithm assigns roles to ports based on their function in the active topology.
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Root : Forwarding port elected for the spanning-tree topology.
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Designated : Forwarding port elected for every switched LAN segment.
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Alternate : Blocked port providing an alternate path to the root bridge.
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Backup : Blocked port in a loopback configuration.
The switch that has all of its ports as the designated role or as the backup role is the root switch. The switch that has at least one of its ports in the designated role is called the designated switch.
Spanning Tree forces redundant data paths into a standby (blocked) state. If a network segment in the spanning tree fails and a redundant path exists, the Spanning Tree algorithm recalculates the spanning-tree topology and activates the standby path.
Switches send and receive Spanning Tree frames, known as bridge protocol data units (BPDUs), at regular intervals. Switches do not forward these frames; instead, they use them to construct a loop-free path. BPDUs contain information about the sending switch and its ports, including the switch MAC address, switch priority, port priority, and path cost. Spanning Tree uses this information to elect the root switch and root port for the switched network, as well as the root port and designated port for each switched segment.