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Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
Chapter 7 Configuring Spanning Tree
Using PVST+
Port Vlan Port-State Cost Prio Portfast Channel_id
------------------------ ---- ------------- --------- ---- -------- ----------
1/1 1 not-connected 4 32 disabled 0
1/2 1 not-connected 4 32 disabled 0
2/1 1 not-connected 100 32 disabled 0
2/2 1 not-connected 100 32 disabled 0
2/3 1 forwarding 19 16 disabled 0
2/4 1 not-connected 100 32 disabled 0
Configuring the 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 that are associated with all the ports to have
the same path cost default set.
There are two default port cost modes available–short and long.
•
The short mode has these parameters:
–
Portcost.
–
Portvlancost.
–
When you enable UplinkFast, the actual cost is incremented by 3000.
•
The long mode has these parameters:
–
Portcost.
–
Portvlancost.
–
When you enable UplinkFast, the actual cost is incremented by 10,000,000.
–
EtherChannel computes the cost of a bundle using the formula,
AVERAGE_COST/NUM_PORT.
The default port cost mode in PVST+ is short. For port speeds of 10 Gb and greater, you must set the
default port cost mode to long.
To change the default port cost mode, perform this task in privileged 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 the 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 that are attached
to faster media (such as full duplex) and higher numbers to ports that are attached to slower media. The
default cost differs for different media.
You can set a cost value from 1
–
65535.
Task
Command
Configure the default port cost mode.
set spantree defaultcostmode {short | long}