Understanding How Bridge Protocol Data Units Work; Calculating And Assigning Port Costs - Cisco WS-C6506 Software Manual

Understanding How Spanning Tree Protocols Work
For example, assume that a port on Switch B is a fiber-optic link. Also, another port on Switch B (an
unshielded twisted-pair [UTP] link) is the root port. Network traffic might be more efficient over the
high-speed fiber-optic link. By changing the Port Priority parameter for the fiber-optic port to a higher
priority (lower numerical value) than the UTP port, the fiber-optic port becomes the root port. You could
also accomplish this scenario by changing the Port Cost parameter for the fiber-optic port to a lower value
than that of the UTP port.

Understanding How Bridge Protocol Data Units Work

The BPDUs contain configuration information about the transmitting switch and its ports, including the
switch and port MAC addresses, switch priority, port priority, and port cost. Each configuration BPDU
contains this information:
•
•
•
The switch sends configuration BPDUs to communicate with and compute the spanning-tree topology.
A MAC frame conveying a BPDU sends the switch group address to the destination address field. All
switches that are connected to the LAN on which the frame is transmitted receive the BPDU. The BPDUs
are not directly forwarded by the switch, but the receiving switch uses the information in the frame to
calculate a BPDU and, if the topology changes, initiates a BPDU transmission.
A BPDU exchange results in the following:
•
•
•
•
•

Calculating and Assigning Port Costs

By calculating and assigning the port cost of the switch ports, you can ensure that the shortest (lowest
cost) distance to the root switch is used to transmit data. You can calculate and assign the lower path cost
values (port costs) to the higher bandwidth ports by using either the short method (which is the default)
or the long method. The short method uses a 16-bit format that yields values from 1 to 65535. The long
method uses a 32-bit format that yields values in the range of 1 to 200,000,000. For more information
on setting the default cost mode, see the
page
You should configure all switches in your network to use the same method for calculating the port cost.
Note
The short method is used to calculate the port cost unless you specify that the long method be used. You
can specify the calculation method using the CLI.
Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
7-4
The unique identifier of the switch that the transmitting switch believes to be the root switch
The cost of the path to the root from the transmitting port
The identifier of the transmitting port
One switch is elected as the root switch.
The shortest distance to the root switch is calculated for each switch.
A designated switch is selected. This is the switch that is closest to the root switch through which
frames will be forwarded to the root.
A port for each switch is selected. This is the port that provides the best path from the switch to the
root switch.
The ports included in the STP are selected.
7-29.
"Configuring the PVST+ Default Port Cost Mode" section on
Chapter 7 Configuring Spanning Tree
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