Scaling the Spanning Tree Protocol
193
Designated bridge has priority 8192, address
00E0.3488.FC00
Designated port is 2, path cost 0
Timers: message age 10, forward delay 7, hold 1
Port FastEthernet 0/27 of VLAN1 is Blocking
Port path cost 20, Port priority 128
Designated root has priority 8192, address
00E0.3488.FC00
Designated bridge has priority 8192, address
00E0.3488.FC00
Designated port is 1, path cost 0
Timers: message age 10, forward delay 7, hold 1
1900A#
In the preceding switch output, notice that port 0/26 is now forwarding
and port f0/27 is now blocking. In the output, the port path cost is 10 for f0/
26 and 20 for f0/27. This is a pretty simple and straightforward configura-
tion, and our network never went down. However, caution should be used
when changing the port costs in a real production network because you can
cause havoc in a network if the configuration is not thought out carefully.
Setting the Port Priority
Another option you can use to help the switch determine the path selection
that STP uses in your network is to set the port priorities. Remember, this
only influences STP; it doesn't demand that STP do anything. However,
between setting the port cost and priority, STP should always make your
path selection.
The port priority and port cost configurations work similarly. The port
with the lowest port priority will forward frames for all VLANs. The com-
mand to set a port priority is set spantree portpri:
Todd5000> (enable) set spantree portpri ?
Usage: set spantree portpri <mod_num/port_num> <priority>
set spantree portpri <trcrf> <trcrf_priority>
(priority = 0..63, trcrf_priority = 0..7)
Todd5000> (enable)
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