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(1) Selecting the root bridge by bridge IDs
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) Designing communication paths
After a root bridge is determined, the communication paths from each designated bridge to
the root bridge are determined.
(a) Selecting the root port based on 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 that have slow ports over paths
than have fast ports, change the configuration to design paths for which communication is
performed.
(b) Selecting designated ports and non-designated ports
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.
19.1.6 STP compatibility mode
(1) Overview
If the Switch uses a rapid Spanning Tree Protocol and the partner switch uses a Spanning
Tree Protocol, the Switch's port connected to the partner switch runs in STP compatibility
mode.
Under STP compatibility mode operation, high-speed transitions are not performed on the
19 Spanning Tree Protocol
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