Cisco WS-C6506 Software Manual page 147

Chapter 7 Configuring Spanning Tree
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.
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.
Table 7-1
Port Speed
10 Mbps
100 Mbps
1 Gbps
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.
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.
Table 7-2
Port Speed
≤ 100 kbps
1 Mbps
10 Mbps
100 Mbps
1 Gbps
10 Gbps
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.
OL-8978-04
Default Port Cost Values Using the Short Method
Default Cost Value
100
19
4
Default Port Cost Values Using the Long Method
Recommended Value
200000000
20000000
2000000
200000
20000
2000
Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
Understanding How Spanning Tree Protocols Work
Table 7-1
Default Range
1 to 65535
1 to 65535
1 to 65535
Recommended Range
20000000 to 200000000
2000000 to 200000000
200000 to 20000000
20000 to 2000000
2000 to 200000
200 to 20000
shows the default
Table 7-2 shows
Available Range
1 to 200000000
1 to 200000000
1 to 200000000
1 to 200000000
1 to 200000000
1 to 200000000
7-5