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Path cost is the sum of costs between a
switch and the root. STP calculates path cost based on the media
speed of the links between switches and the port cost of each port
forwarding frames.
If you want to change the path of
frames between a particular switch and the root, carefully calculate
the current path cost, and then change the port costs of the desired
path. Make sure that you calculate the sum of potential alternate
paths in addition to the desired path before making changes. This
ensures a proper assessment of path costs prior to making port-cost
changes and that frames are forwarded over the chosen path ports.
Assign lower numbers to ports
attached to faster media (such as full duplex) and higher numbers to
ports attached to slower media. The possible range is 1 to 65535.
The default differs for different media. Path cost is typically 1000
divided by LAN speed in Mbps.
To change the port cost for a port on
a set-command-based switch, enter the set
spantree portcost mod-num/port-num cost
command in privileged mode.
The following is an example of
entering the set spantree
portcost 2/1 command. (see
Figure)
Switch> (enable) set
spantree portcost 2/1 10
Spantree port 2/1 path cost set to 10.
Verify the port-cost setting by
entering the show spantree
[mod-num/port-num] command
in privileged mode as shown in the Figure. The console returns a
display similar to the previously entered command for verifying port
priority.
To change the port cost for a port on
a Cisco IOS command-based switch, enter the following command in
interface configuration mode
Switch(config-if)#spantree cost
cost-value
where cost-value is a numeric
value within the range of 1 to 65535. To return the path cost for an
interface to the default value, enter the no
spantree cost command in
interface configuration mode.
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