What determines which switch becomes the root bridge in STP?

In Spanning Tree Protocol (STP), the root bridge is determined by the Bridge ID, which comprises two components: the bridge priority and the MAC address of the switch. Each switch has a default bridge priority, typically set to 32768, and the bridge ID is formed by combining this priority with the MAC address of the switch.

The switch with the lowest Bridge ID becomes the root bridge. If multiple switches have the same priority, the MAC address will then be the deciding factor. This hierarchy of criteria ensures a consistent method for electing the root bridge, which plays a crucial role in managing the topology of the network to prevent loops.

In contrast, choosing the switch with the lowest MAC or IP address alone isn't sufficient, as these factors are part of the Bridge ID calculation. Therefore, understanding the concept of Bridge ID and its components is essential for grasping how a root bridge is elected in STP.