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19 Spanning Tree Protocol
(4) Path cost
A value corresponding to the communication speed of each port on a switch is called the
path cost. The total value of the port costs for all intermediate ports from a designated
bridge to the root bridge is called the root path cost. If there are multiple paths to the root
bridge, the root path cost is that of the shortest path.
It is recommended that a lower path cost be specified for a port that has a faster packet
transmission speed (that is, the faster the port, the lower the specified path cost). The
default value of the path cost corresponds to the speed of the port, but can also be changed
in the configuration.
(5) Port identifier
Each port in a switch is identified by a parameter called a port ID. Port IDs are used to
select a communication path when two or more redundant connections exist between two
switches, and the path cost cannot be changed for each port. Note that when redundant
connections are used between two switches, we recommend using link aggregation. Use a
Spanning Tree Protocol to enable redundant connections between switches that do not
support link aggregation.
Port IDs consist of a port priority (4 bits) and a port number (12 bits). The following figure
shows a port ID.
Figure 19-3 Port ID
19.1.5 Designing Spanning Tree topologies
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 19-4 Designing Spanning Tree topologies
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