D = Designated port R = Root port
V through Z = LANs
The first activity in spanning-tree computation is the selection of the root bridge, which is the bridge with the lowest-value bridge identifier. In Figure 20-3, the root bridge is Bridge 1. Next, the root port on all other bridges is determined. A bridge's root port is the port through which the root bridge can be reached with the least aggregate path cost. The value of the least aggregate path cost to the root is called the root path cost.
Finally, designated bridges and their designated ports are determined. A designated bridge is the bridge on each LAN that provides the minimum root path cost. A LAN's designated bridge is the only bridge allowed to forward frames to and from the LAN for which it is the designated bridge. A LAN's designated port is the port that connects it to the designated bridge.
In some cases, two or more bridges can have the same root path cost. For example, in Figure 20-3, Bridges 4 and 5 can both reach Bridge 1 (the root bridge) with a path cost of 10. In this case, the bridge identifiers are used again, this time to determine the designated bridges. Bridge 4's LAN V port is selected over Bridge 5's LAN V port.
Using this process, all but one of the bridges directly connected to each LAN are eliminated, thereby removing all two-LAN loops. The STA also eliminates loops involving more than two LANs, while still preserving connectivity. Figure 20-4 shows the results of applying the STA to the network shown in Figure 20-3. Figure 20-4 shows the tree topology more clearly. Comparing this figure to the pre-spanning-tree figure shows that the STA has placed both Bridge 3's and Bridge 5's ports to LAN
V in standby mode.
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