7-5
Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
OL-8978-04
Chapter 7 Configuring Spanning Tree
Understanding How Spanning Tree Protocols Work
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 the 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.
Calculating the Port Cost Using the Long Method
802.1t assigns 32-bit (long) default port cost values to each port using a formula that is based on the
bandwidth of the port. You can also manually assign port costs between 1–200,000,000. The formula for
obtaining default 32-bit port costs is to divide the bandwidth of the port by 200,000,000.
Table 7-2
shows
the default port cost values that are assigned by the switch and the recommended cost values and ranges
for each type of port when you use the long method to calculate port cost.
Calculating the Port Cost for Aggregate Links
As individual links are added or removed from an aggregate link (port bundle), the bandwidth of the
aggregate link increases or decreases. These changes in bandwidth lead to recalculation of the default
port cost for the aggregated port. Changes to the default port cost or changes resulting from links that
autonegotiate their bandwidth could lead to recalculation of the spanning-tree topology which may not
be desirable, especially if the added or removed link is of little consequence to the bandwidth of the
aggregate link (for example, if a 10-Mbps link is removed from a 10-Gbps aggregate link). Because of
the limitations that are presented by automatically recalculating the topology, 802.1t states that changes
in bandwidth will not result in changes to the cost of the port. The aggregated port will use the same port
cost parameters as a standalone port.
Table 7-1
Default Port Cost Values Using the Short Method
Port Speed
Default Cost Value
Default Range
10 Mbps
100
1 to 65535
100 Mbps
19
1 to 65535
1 Gbps
4
1 to 65535
Table 7-2
Default Port Cost Values Using the Long Method
Port Speed
Recommended Value
Recommended Range
Available Range
≤
100 kbps
200000000
20000000 to 200000000
1 to 200000000
1 Mbps
20000000
2000000 to 200000000
1 to 200000000
10 Mbps
2000000
200000 to 20000000
1 to 200000000
100 Mbps
200000
20000 to 2000000
1 to 200000000
1 Gbps
20000
2000 to 200000
1 to 200000000
10 Gbps
2000
200 to 20000
1 to 200000000
