7-5
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2980G Switches Software Configuration Guide
—
Release 8.1
78-15486-01
Chapter 7 Configuring Spanning Tree
Understanding How STPs Work
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 port
bandwidth. 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 the 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. Recalculation
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 concerned. Therefore, the
aggregated port uses the same port cost parameters as a standalone port.
Understanding Spanning Tree Port States
Topology changes can take place in a switched network due to a link coming up or going down (failing).
When a switch port transitions directly from nonparticipation in the topology to the forwarding state, it
can create temporary data loops. Ports must wait for new topology information to propagate through the
switches in the LAN before they can start forwarding frames. They must also allow the frame lifetime
to expire for frames that have been forwarded using the old topology.
At any given time, each port on a switch using STP is in one of these states:
•
Blocking
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
