Setting the Port Cost

Setting the Port Cost
The parameters in this next set are used to enable the network administrator to influence the
path that spanning tree chooses when setting the port priority, port cost, and path cost.
Cisco does not recommend changing these settings unless it’s absolutely necessary. However,
the best way to get a good understanding of how the STP works is by changing the defaults. We
do not recommend trying any of this on a production network unless you have permission from
the network manager, who understands that you can bring the network down by doing so.
By changing the port cost, you can change the port ID, which means it can be a more
desirable port to STP. Remember that STP uses the port ID only if there is more than one
path to the root bridge and they are of equal cost. Path cost is the sum of the costs between
Scaling the Spanning Tree Protocol 511
a switch and the root bridge. The STP calculates the path cost based on the media speed of
the links between the switch and the port cost of each port forwarding the frames. In the
hands-on lab on the accompanying CD, both links are 100Mbps, so the port ID is important
and will be used.
To change the path used between a switch and the root bridge, first calculate the current path
cost. Then change the port cost of the port you want to use, making sure that you keep in mind
the alternate paths if the primary path fails before making any changes to your switch. Remember
that ports with a lower port cost are more likely to be chosen; this doesn’t mean they always
will be chosen.
To change the port cost of a port on a 4000 series switch, use the set spantree portcost
command:
Terry_4000> (enable) set spantree portcost ?
Usage: set spantree portcost <mod_num/port_num> <cost>
set spantree portcost <trcrf> <cost>
(cost = 1..65535)
The parameters to set the cost of a port are the module and port number and the cost you
want to configure. The following example shows how to set the port cost on port 1/1 from the
default of 19 to 10:
Terry_4000> (enable) set spantree portcost 1/1 10
Spantree port 1/1 path cost set to 10.
You would verify the change with the show spantree command. However, because both
ports are in forwarding mode, the preceding command will not change the switch’s STP parameters.
Notice in the following switch output that both ports are forwarding, but the costs of the
ports are different:

Port Vlan Port-State Cost Priority Fast-Start
--------- ---- ------------- ----- -------- ----------
1/1 1 forwarding 10 32 disabled
1/2 1 forwarding 19 32 disabled
Remember that a root switch will be forwarding on all active ports, so the port IDs are irrelevant
to the switch. However, the 2950 must then choose a port to perform blocking on the
interface with the lowest cost.
To change the port cost on an IOS-based switch, use the spanning-tree cost interface
command. The cost value can be any number from 1 to 200000000; however, you cannot make
it less than the path cost of both links. What you need to do is to raise the port priority of the
port that we don’t want STP to use for forwarding. Notice in the following example that we
change the cost of port fa0/24 to 20. This should make the fa0/24 port a more desirable path:
Terry_2950#conf t
Enter configuration commands, one per line. End with CNTL/Z.
512 Chapter 16  Using Spanning Tree with VLANs
Terry_2950(config)#interface fa0/24
Terry_2950(config-if)#spanning-tree ?
bpdufilter Don't send or receive BPDUs on this interface
bpduguard Don't accept BPDUs on this interface
cost Change an interface's spanning tree port path cost
guard Change an interface's spanning tree guard mode
link-type Specify a link type for spanning tree protocol use
port-priority Change an interface's spanning tree port priority
portfast Enable an interface to move directly to forwarding on link up
stack-port Enable stack port
vlan VLAN Switch Spanning Tree
Terry_2950(config-if)#spanning-tree cost ?
<1-200000000> port path cost
Terry_2950(config-if)#spanning-tree cost 20
Terry_2950(config-if)#^Z
To verify the port priorities, use the show spanning-tree command:
Terry_2950#show spanning-tree


VLAN0001
Spanning tree enabled protocol ieee
[Output cut]
Interface Port ID Designated Port ID
Name Prio.Nbr Cost Sts Cost Bridge ID Prio.Nbr
---------------- -------- --------------- -------------------- --------
Fa0/1 128.1 5 FWD 0 1 00b0.6414.1180 128.1
Fa0/24 128.24 20 BLK 0 1 00b0.6414.1180 128.12
In the preceding switch output, notice that port fa0/1 is forwarding and port fa0/24 is now
blocking. In the output, the port path cost is 5 for port fa0/1 and 20 for port fa0/24. This is a
pretty simple and straightforward configuration and worked fine, but the network suffered
downtime due to convergence, so caution should be used when changing the port costs in a real
production network. Also, you need to plan your final topology, because you can cause havoc
in a network if the configuration is not thought out carefully. The port costs are propagated in
the BPDUs, so a small change on one switch can affect how spanning tree chooses the various
ports on a switch a few cable segments away.


You can get a good idea of the delays associated with spanning tree convergence
if you try this out for yourself. Immediately after making the changes to
the port cost, enter the show spanning-tree command on the 2950 switch. If
you keep repeating the command, you will see the switch going through the
blocking, listening, and learning modes on the way to forwarding. You can time
the process with your watch.