Configuring Path Cost , on page 231
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 decides 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
Consolidated Platform Configuration Guide, Cisco IOS Release 15.2(4)E (Catalyst 2960-X Switches)
216
Information About Spanning Tree Protocol
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