Cisco NCS 4200 Series Configuration Manual page 47

Configuring PVST+ and RPVST+
• 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.
A port participating in spanning tree 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
The figure below shows how an interface moves through the states.
Figure 5: Spanning-Tree Interface States
Spanning tree is not enabled by default. Once the spanning tree mode is selected, each VLAN on ports goes
through the blocking state and the transitionary states of listening and learning. Spanning tree stabilizes each
interface at the forwarding or blocking state.
When the spanning-tree algorithm places a Layer 2 spanning-tree interface in the forwarding state, this process
occurs:
1 The interface is in the listening state while spanning tree waits for protocol information to transition the
interface to the blocking state.
2 While spanning tree waits the forward-delay timer to expire, it moves the interface to the learning state
and resets the forward-delay timer.
3 In the learning state, the interface continues to block frame forwarding as the switch learns end-station
location information for the forwarding database.
LAN Switching Configuration Guide, Cisco IOS XE Everest 16.6.1 (Cisco NCS 4200 Series)
Spanning-Tree Interface States
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