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Chapter 7
Configuring Spanning Tree
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Configuring PVST+ Default Port Cost Mode
If any switch in your network is using a port speed of 10 Gb or over and the network is using PVST+
spanning tree mode, all switches in the network must have the same path cost defaults. You can enter
the set spantree defaultcostmode command to force all VLANs associated with all the ports to have
the same pathcost default set.
There are two default port cost modes available – short and long.
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The default port cost mode is set to short in PVST+ mode. For port speeds of 10 Gb and greater, the
default port cost mode must be set to long.
To change the default port cost mode, perform this task in privileged mode:
Task
Set the default port cost mode.
This example shows how to configure the default port cost mode:
Console> (enable) set spantree defaultcostmode long
Portcost and portvlancost set to use long format default values.
Console> (enable)
Configuring PVST+ Port VLAN Cost
You can configure the port cost for a port on a per-VLAN basis. Ports with a lower port VLAN cost are
more likely to be chosen to forward frames. You should assign lower numbers to ports attached to faster
media (such as full duplex) and higher numbers to ports attached to slower media. The default cost
differs for different media.
You can set a cost value from 1 to 65535.
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The short mode has these parameters:
Portcost
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Portvlancost
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When uplinkfast is enabled, the actual cost is incremented by 3000
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The long mode has these parameters:
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Portcost
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Portvlancost
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When uplinkfast is enabled, the actual cost is incremented by 10,000,000
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EtherChannel computes the cost of a bundle using the formula,
AVERAGE_COST/NUM_PORT
Software Configuration Guide—Catalyst 4000 Family, Catalyst 2948G, Catalyst 2980G, Releases 6.3 and 6.4
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Command
set spantree defaultcostmode {short | long}
Using PVST+
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Troubleshooting
