To avoid loops we need a tree topology consisting of a root, a succession of branches and then leaves. The leaves represent end stations, and there is one and only one path from a leaf to another leaf. Therefore, the tree is free of loops that can cause havoc in a network. The other requirement is that all leaves are connected. There are no isolated segments. Another term for this topology is distributed star. Within our tree structure will be a series of bridges used to connect the branches and leaves. There are two types. The root bridge is the main one of interest because it has a special assignment and there is only one within a network. The other bridges (that are to be used) are all designated bridges and there could be many within the network. To have a tree topology, you need bridges with more than two ports.

Port Designations

Although bridges do not need MAC addresses to operate, a MAC address is needed to identify bridges in the Spanning Tree Protocol. Besides a MAC address for each bridge, each port on each bridge must be identified. For bridges, a unique 64-bit bridge identifier is assigned by appending a 16-bit priority field in front of a unique 48-bit MAC address resulting in a Bridge ID. The MAC address comes from the bridge vendor, but the priority field can be set by the user. The default priority value of 0x8000 is in the middle of the priority range. If the user fails to assign priority values, the bridges will still have unique assignments. This is important since the bridge with the lowest numerical bridge identifier will become the root bridge. All other bridges have the possibility of becoming designated bridges.

Similarly, a 16-bit "port identifier" exists consisting of an 8-bit port address preceded by an 8-bit priority field. Again, the user sets the priority field while the bridge vendor sets the port addresses usually beginning with one for port one and so on. The default priority field is 0x80. Now we have all the bridges and ports identified including the root bridge.

To avoid loops, there is one and only one bridge that is responsible for forwarding messages from the direction of the root towards branches, which we will call links. If there is only one path from the root to a link where end stations (leaves) attach, there will be no loops. We need a forwarding policy, which is called the Spanning Tree Protocol. To implement this policy, we need to assign each port to become either a designated port or a root port. A designated port is a port that forwards traffic away from the root and towards the leaves. A root port carries traffic back to the root bridge with the further requirement that no more than one root port exists on any one designated bridge. Looking at Figure 2, notice that the root bridge (R) has all designated ports (because it is the root) while the designated bridges have root ports when connected to the root bridge or when the direction of flow is towards the root.

Figure 2 — The addition of link U creates a redundant path. Bridge R has the lowest bridge ID and is, therefore, the root.