Cisco WS-C2980G-A Software Guide - Page 100

Understanding How STPs Work Chapter 7 Configuring Spanning Tree Understanding BPDUs BPDUs contain configuration information about the transmitting switch and its ports, including switch and port MAC addresses, switch priority, port priority, and port cost. Each configuration BPDU contains this information: • 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 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 connected to the LAN on which the frame is transmitted receive the BPDU. BPDUs are not directly forwarded by the switch, but the receiving switch uses the information in the frame to calculate a BPDU. If the topology changes, the receiving switch initiates a BPDU transmission. A BPDU exchange results in the following: • 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. • Ports included in the STP are selected. 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 lower path cost values (port costs) to 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 1-65535. The long method uses a 32-bit format that yields values from 1-200,000,000. For more information on setting the default cost mode, see the "Configuring the PVST+ Default Port Cost Mode" section on page 7-26. Note You should configure all switches in your network to use the same method for calculating port cost. The short method (default) will be used to calculate the port cost unless you specify the long method. You can specify the calculation method using the CLI. Calculating the Port Cost Using the Short Method The IEEE 802.1D specification assigns 16-bit (short) default port cost values to each port that is based on bandwidth. You can also manually assign port costs between 1-65535. The 16-bit values are only used for ports that have not been specifically configured for port cost. Table 7-1 shows the default port cost values that are assigned by the switch for each type of port when you use the short method to calculate the port cost. Catalyst 4500 Series, Catalyst 2948G, Catalyst 2980G Switches Software Configuration Guide-Release 8.1 7-4 78-15486-01