Allied Telesis FORMULA 8200 - Manual (Page 38)

Virtual LANs
3-6
Spanning Tree Spanning Tree is a configuration algorithm and protocol that ensures
that no data loops exist within a single broadcast domain. For
example, Figure 3-3 shows bridges 1 and 2 in a loop; in this
configuration without Spanning Tree, the network is unusable.
Figure 3-3: Data Looping
When Spanning Tree is implemented, redundant bridge ports are
blocked and looping is eliminated, as shown in Figure 3-4.
Figure 3-4: Spanning Tree Eliminating Data Looping
By blocking the port at Bridge 2, data can pass to all three segments.
In this example, either Bridge 1 or Bridge 3 becomes the “root”
bridge, depending on priority or MAC address, and the remaining
bridge then becomes a “standby” bridge, ready to function if a failure
should occur in the Bridge 1.
forwarding
forwarding
data looping
forwarding
forwarding
forwarding
forwarding
Bridge 1
Bridge 3
Bridge 2
VLAN 1
VLAN 2
VLAN 3
3011
not forwarding
Bridge 1
Bridge 3
Bridge 2
VLAN 1
VLAN 2
VLAN 3
3012
blocking