What Spanning Tree Does
STP/RSTP prevents loops by placing each switch port in either a forwarding state or a block-
ing state. Interfaces in the forwarding state act as normal, forwarding and receiving frames.
However, interfaces in a blocking state do not process any frames except STP/RSTP mes-
sages (and some other overhead messages). Interfaces that block do not forward user frames,
do not learn MAC addresses of received frames, and do not process received user frames.
Figure 9-2 shows a simple STP/RSTP tree that solves the problem shown in Figure 9-1 by
placing one port on SW3 in the blocking state.
Gi0/1
Gi0/1
Gi0/2
Gi0/2
Gi0/1
Gi0/2
Fa0/13
0200.3333.3333
Bob
Larry
Fa0/11
Fa0/12
SW2
SW3
SW1
Archie
1
2
3
3
4
5
4
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