28.1.5 Topology design of spanning tree

The topology of a Spanning Tree Protocol is based on the bridge ID and path cost. The figure below shows the basic procedures for designing a topology. In the example configuration in the figure, two core switches are used for redundancy, placed to handle terminals as edge switches.

Figure 28-4: Topology design of spanning tree

<Structure of this section>

(1) Selecting the Root Bridge by Bridge Identifier

The switch with the lowest bridge ID is chosen as the root bridge. Normally, you set the bridge priority of the switch that you want to be the root bridge to the lowest value (highest priority). In the example in the figure, Switch A is the root bridge, and Switch B and Switch C are designated bridges.

Note that Switch B will become the alternate root bridge if a fault occurs on the root bridge. Switch C is set as the lowest priority.

For the design of a Spanning Tree topology, we recommend configurations that follow the example in the figure of setting the switch handling the network core as the root bridge and using alternate root bridges to make the core redundant.

(2) Design of communication paths

After a root bridge is determined, the communication paths from each designated bridge to the root bridge are determined.

(a) Route Port Selection with Path Cost

For Switch B and Switch C, the path to the root bridge is determined by finding the lowest root path cost value. In the example in the figure, the path cost for all ports is 200000. Among the directly connected ports, the one with the lowest root path cost is chosen as the root port.

The root path cost of a path from a designated bridge to the root bridge is calculated by comparing the total path cost of the outgoing ports bound for the root bridge for each switch. For example, because the path cost of the path passing through Switch B for Switch C is 400000, it is not chosen for the root port.

The default cost for a path is the smallest value, which is based on the fastest port speed. In addition, the root port is determined by comparing root path costs. Therefore, you normally do not need to make changes to path costs to prioritize the use of paths with fast ports or the minimum of intermediate switches. To prioritize paths than have slow ports over paths than have fast ports, change the configuration to design paths for which communication is performed.

(b) Selection of designated port and non-designated port

Ports other than the root port are used for the connection between Switch B and Switch C. One or more of these ports are non-designated ports and are placed in Blocking status. This is how Spanning Tree Protocols use the Blocking status on a given side to prevent loops.

Designated ports and non-designated ports are chosen as follows:

The port on the switch with the lowest root path cost between switches is the designated port, and ports on higher cost switches are non-designated ports.
If root path costs are the same, the port on the switch that has the smaller bridge ID is the designated port, and ports on switches that have larger IDs are non-designated ports.

In the example in the figure, the root path costs are the same. According to the bridge priority, Switch B has the designated port and Switch C has the non-designated port, which is placed in Blocking status. To change the port of Switch B to Blocking status, set the path costs so that the root path cost of Switch B increases.