Specifications

9-5
Cisco MWR 2941 Mobile Wireless Edge Router Release 3.5 Software Configuration Guide, Cisco IOS Release 15.1(3)MR
OL-26895-01
Chapter 9 Configuring STP
Understanding Spanning-Tree Features
Figure 9-1 illustrates how an interface moves through the states.
Figure 9-1 Spanning-Tree Interface States
When you power up the router, spanning tree is enabled by default, and every NNI in the
Cisco MWR 2941 router (and every ENI on which STP has been enabled), as well as any other port in
other switches in the VLAN or network that are participating in spanning tree, goes through the blocking
state and the transitory 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 router learns
end-station location information for the forwarding database.
4. When the forward-delay timer expires, spanning tree moves the interface to the forwarding state,
where both learning and frame forwarding are enabled.
Blocking State
A Layer 2 interface in the blocking state does not participate in frame forwarding. After initialization, a
BPDU is sent to each switch interface, or to each switch STP port. A switch initially functions as the
root until it exchanges BPDUs with other switches. This exchange establishes which switch in the
network is the root or root switch. If there is only one switch in the network, no exchange occurs, the
forward-delay timer expires, and the interface moves to the listening state. An interface participating in
spanning tree always enters the blocking state after switch initialization.
An interface in the blocking state performs these functions:
• Discards frames received on the interface
• Discards frames switched from another interface for forwarding
Power-on
initialization
Blocking
state
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Listening
state
Disabled
state
Learning
state
Forwarding
state