Cisco Cisco Aironet 340 Ethernet Bridges Guia Do Utilizador

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Using the Spanning-Tree Protocol   8 - 11
Figure 8.2 provides a sample configuration in which it would be useful 
to change the root bridge.
Bridge 4 is the root with the spanning tree shown by the thick line. STP 
has been disabled to the port on Bridge 1 to prevent a loop.
Figure 8.2 -  Non-Optimal Choice of Root Bridge
LAN A
LAN C
LAN D
LAN B
Bridge 1
Bridge 3
Bridge 4 (Root)
Port disabled
by STP
Bridge 2