User's Manual

6-7
Cisco 3200 Series Wireless MIC Software Configuration Guide
OL-7734-02
Chapter 6 Configuring Spanning Tree Protocol
Understanding Spanning Tree Protocol
When the spanning-tree algorithm places a Layer 2 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 bridge 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
An interface in the blocking state does not participate in frame forwarding. After initialization, a BPDU
is sent to the bridge’s Ethernet and radio ports. A bridge initially functions as the spanning-tree root until
it exchanges BPDUs with other bridges. This exchange establishes which bridge in the network is the
spanning-tree root. If there is only one bridge in the network, no exchange occurs, the forward-delay
timer expires, and the interfaces move to the listening state. An interface always enters the blocking state
when you enable STP.
An interface in the blocking state performs as follows:
• Discards frames received on the port
• Does not learn addresses
• Receives BPDUs
Note If a port is blocked, some broadcast or multicast packets can reach a forwarding port on the bridge and
cause the bridging logic to switch the blocked port into listening state momentarily before the packets
are dropped at the blocked port.
Listening State
The listening state is the first state an interface enters after the blocking state. The interface enters this
state when STP determines that the interface should participate in frame forwarding.
An interface in the listening state performs as follows:
• Discards frames received on the port
• Does not learn addresses
• Receives BPDUs