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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter10 Configuring STP
Understanding Basic STP Features
STP Interface States
Propagation delays can occur when protocol information pass es thro ugh a sw it ched L AN. A s a resul t,
topology changes can take place at different times and at different places in a swi tch ed net wor k. Whe n
a Layer 2 interface transitions directly from nonparticipation in the spanning-tree topology to the
forwarding state, it can create temporary data loops. Interfaces must wait for new topology information
to propagate through the switched LAN before startin g to forw ard fram es. They must allow the fr ame
lifetime to expire for forwarded frames that have used the old topology.
Each Layer 2 interface on a switch using STP exists in one of these states:
•Blocking—The Layer 2 interface does not participate in fr ame fo rw ardi ng.
•Listening—the first transitional state after the blocking state when STP determines th at th e La ye r2
interface should participate in frame forwarding.
•Learning—The Layer2 interface prepares to participate in frame forwarding.
•Forwarding—The Layer 2 interface forwards frames.
•Disabled—The Layer 2 interface is not participating in STP because of a shutd own port, no link on
the port, or no spanning-tree instance running on the po rt.
A Layer 2 interface moves through these states:
•From initialization to blocking
•From blocking to listening or to disabled
•From listening to learning or to disabled
•From learning to forwarding or to disabled
•From forwarding to disabled
Figure 10-2 illustrates how a Layer 2 interface moves through the states.
Figure10-2 Spanning Tree Layer 2 Interface States
When you power up the switch, STP is enabled by default, and every interface in the switch, VLAN, or
network goes through the blocking state and the transitory states of listening and learning. Spann ing tree
stabilizes each Layer 2 interface at the forwarding or blocking state.
Power-on
initialization
Blocking
state
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Listening
state Disabled
state
Learning
state
Forwarding
state