The first critical concept in understanding Spanning Tree is that of the Root Bridge. In any Spanning Tree instance, there is only one Root Bridge, and it must be elected. The Root Bridge is elected in the initial exchange of BPDUs between bridging devices. But how does the Root Bridge get elected? That’s simple. In networks running STP, every bridge has a priority value associated with it. By default, the priority of all bridges is 32,768, unless changed by an administrator. The bridge with the highest priority gets to be the Root Bridge. But wait – you’ll need to remember that the highest priority is the bridge with the lowest priority value. That is, a bridge priority of 1000 would beat the default priority of 32,768.
You have probably never touched bridge priorities. So if all the priorities are equal, who wins? That answer is the bridge with the lowest MAC address. All BPDUs contain a field called the Bridge ID (BID), which is actually made up of both the bridge’s priority and its MAC address. In cases where all priorities are equal, the bridge with the lowest MAC address gets to be the Root Bridge. Consider Figure the figure below, where Bridge A will become the Root Bridge, based on its MAC address.
Figure: Root Bridge election using priorities and MAC addresses.
So why should you care about a Root Bridge? The answer is that all other bridging devices need to calculate a path to the Root Bridge in order to make decisions on which links will be used, and which will not. By calculating the distance to the Root Bridge, not only does STP work to build a loop-free topology, but also one that will have the shortest distance between devices. In the case of STP, “shortest” refers to the path that has the lowest aggregate cost to the root. We’ll discuss aggregate costs in just a moment.
Tip: When bridge priorities are equal, the switch with the lowest MAC address will become the root bridge.
