This topic explains the actions that result from a BPDU exchange in a switched network, including root switch election, port selection, and path blocking in the Spanning Tree topology.
A BPDU exchange results in these actions in a switched network:
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One switch in the network is elected as the root switch, the logical center of the Spanning Tree topology in a switched network.
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For each VLAN, the switch with the highest switch priority (the lowest numerical priority value) is elected as the root switch. If all switches are configured with the default priority (32768), the switch with the lowest MAC address in the VLAN becomes the root switch. The switch priority value is stored in the most significant bits of the bridge ID.
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A root port is selected for each switch (except the root switch). This port provides the best path (lowest cost) when the switch forwards packets to the root switch.
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The shortest distance to the root switch is calculated for each switch based on path cost.
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A designated switch for each LAN segment is selected. The designated switch incurs the lowest path cost when forwarding packets from that LAN to the root switch. The port that connects the designated switch to the LAN is called the designated port.
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All paths that are not needed to reach the root switch from anywhere in the switched network are placed in the Spanning Tree blocking mode.