By default, following initialisation, all switches initially assume that they are the Root of the Spanning Tree, until they exchange BPDUs with other switches. When switches exchange BPDUs, an election is held and the switch with the lowest Bridge ID in the network is elected the STP Root Bridge. If two or more switches have the same priority, the switch with the lowest order MAC address is chosen. This concept is illustrated in Figure 31.4 below:
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