Specifications

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Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
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Chapter 12 Configuring STP
Understanding Spanning-Tree Features
Spanning-Tree 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
an 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 spanning tree exists in one of these states:
• Blocking—The interface does not participate in frame forwarding.
• Listening—The first transitional state after the blocking state when the spanning tree determines
that the interface should participate in frame forwarding.
• Learning—The interface prepares to participate in frame forwarding.
• Forwarding—The interface forwards frames.
• Disabled—The interface is not participating in spanning tree because of a shutdown port, no link on
the port, or no spanning-tree instance running on the port.
An 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 12-2 illustrates how an interface moves through the states.
Figure 12-2 Spanning-Tree 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 interface at the forwarding or blocking state.
Power-on
initialization
Blocking
state
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Listening
state
Disabled
state
Learning
state
Forwarding
state