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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter 10 Configuring STP
Understanding Basic STP Features
STP Interface States
Propagation delays can occur when protocol information passes through a switched LAN. As a result,
topology changes can take place at different times and at different places in a switched network. When
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 starting to forward frames. They must allow the frame
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 frame forwarding.
•
Listening—the first transitional state after the blocking state when STP determines that the Layer 2
interface should participate in frame forwarding.
•
Learning—The Layer 2 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 shutdown port, no link on
the port, or no spanning-tree instance running on the port.
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.
Figure 10-2 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. Spanning 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
