Cisco Systems ME3400G2CSA Blocking State, Listening State, Learning State

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Cisco ME 3400 EthernetAccess Switch SoftwareConfiguration Guide
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Chapter14 Configuring STP
Understanding Spanning-Tree Features
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.
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. Af ter initialization, a
BPDU is sent to each switch interface, or to each switch NNI. 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 forwardi ng
•Does not learn addresses
•Receives BPDUs
Listening State
The listening state is the first state a Layer 2 i nterface enter s after the blo cking s tate. T he interf ace e nters
this state when the spanning tree decides that the interface s hould pa rtic ipa te in f ra me f orward in g.
An interface in the listening state performs these functions:
•Discards frames received on the interface
•Discards frames switched from another interface for forwardi ng
•Does not learn addresses
•Receives BPDUs
Learning State
A Layer 2 interface in the learning state prepares to participate in f rame for warding . The inte rface enters
the learning state from the listening state.
An interface in the learning state performs these functions:
•Discards frames received on the interface
•Discards frames switched from another interface for forwardi ng
•Learns addresses
•Receives BPDUs