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Chapter 7
Configuring Spanning Tree
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.
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.
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. Therefore, the aggregated port will use
the same port cost parameters as a standalone port.
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.
With IOS Release 12.1.(1)E or later releases, the ARP on STP Topology Change Notification feature
Note
ensures that excessive flooding does not occur when the MSFC receives a topology change notification
(TCN) from the supervisor engine. The feature causes the MSFC to send ARP requests for all the ARP
entries belonging to the VLAN interface where the TCN is received. When the ARP replies come back,
the PFC learns the MAC entries which were lost as a result of the topology change. Learning the entries
immediately following a topology change prevents excessive flooding later. There is no configuration
required on the MSFC. This feature works with supervisor engine software release 5.4(2) or later
releases.
At any given time each port on a switch using STP is in one of these states:
78-12647-02
Default Port Cost Values Using the Long Method
Recommended Value
200000000
20000000
2000000
200000
20000
2000
Software Configuration Guide—Catalyst 4000 Family, Catalyst 2948G, Catalyst 2980G, Releases 6.3 and 6.4
How Spanning Tree Protocols Work
Recommended Range
20000000 to 200000000
2000000 to 200000000
200000 to 20000000
20000 to 2000000
2000 to 200000
200 to 20000
Table 8-2
Available Range
1 to 200000000
1 to 200000000
1 to 200000000
1 to 200000000
1 to 200000000
1 to 200000000
7-5
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