Blocking State; Listening State; Learning State - Cisco ME 3400G-2CS - Ethernet Access Switch Software Configuration Manual

Understanding Spanning-Tree Features
When the spanning-tree algorithm places a Layer 2 spanning-tree interface in the forwarding state, this
process occurs:
1.
2.
3.
4.

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 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:
•
•
•
•

Listening State

The listening state is the first state a Layer 2 interface enters after the blocking state. The interface enters
this state when the spanning tree decides that the interface should participate in frame forwarding.
An interface in the listening state performs these functions:
•
•
•
•

Learning State

A Layer 2 interface in the learning state prepares to participate in frame forwarding. The interface enters
the learning state from the listening state.
An interface in the learning state performs these functions:
•
•
•
•
Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
14-6
The interface is in the listening state while spanning tree waits for protocol information to transition
the interface to the blocking state.
While spanning tree waits the forward-delay timer to expire, it moves the interface to the learning
state and resets the forward-delay timer.
In the learning state, the interface continues to block frame forwarding as the switch learns
end-station location information for the forwarding database.
When the forward-delay timer expires, spanning tree moves the interface to the forwarding state,
where both learning and frame forwarding are enabled.
Discards frames received on the interface
Discards frames switched from another interface for forwarding
Does not learn addresses
Receives BPDUs
Discards frames received on the interface
Discards frames switched from another interface for forwarding
Does not learn addresses
Receives BPDUs
Discards frames received on the interface
Discards frames switched from another interface for forwarding
Learns addresses
Receives BPDUs
Chapter 14 Configuring STP
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