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Chapter 5

Using Spanning Tree with VLANs

The possible port priority range is from 0 to 63, where the default is 32.

If all ports have the same priority, then the port with the lowest port number
will forward frames. For example, 2/1 is lower than 2/2. In the following
example, the 5000 switch priority for port 1/1 is set to 20:

Todd5000> (enable) set spantree portpri 1/1 20

Bridge port 1/1 port priority set to 20.

Todd5000> (enable)

Once you change your port priority, you can verify the configuration with

the show spantree 1/1 command:

Todd5000> (enable) sh spantree 1/1

Port Vlan Port-State Cost Priority Fast-Start

--------- ---- ------------- ----- -------- ----------

1/1 1 forwarding 10 20 disabled

1/1 2 forwarding 10 20 disabled

1/1 3 forwarding 10 20 disabled

1/1 4 forwarding 10 20 disabled

1/1 1003 not-connected 10 20 disabled

1/1 1005 not-connected 10 4 disabled

Todd5000> (enable)

Notice that, because port 1/1 is a trunked port, all VLAN priorities were

changed on that port. Also notice in the following output that the priority is
20 for 1/1, but the default of 32 is set for 1/2:

Todd5000> (enable)Sh spantree

[output cut]

Port Vlan Port-State Cost Priority Fast-Start

--------- ---- ------------- ----- -------- ----------

1/1 1 forwarding 10 20 disabled

1/2 1 forwarding 19 32 disabled

You can go one step further and set the port priority on a per-VLAN

basis. The port with the lowest priority will forward frames for the VLAN
for which you've set the priority. Again, if all the ports have the same prior-
ity, the lowest port number wins and begins forwarding frames.

There is an advantage to setting the port priority per VLAN. If you have

a network with parallel paths, STP will stop at least one link from forward-
ing frames so a network loop will not occur. All traffic would then have to

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