Cisco IE 4000, Software Configuration Manual

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Configuring STP
Information About Configuring STP
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. 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 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

Does not learn addresses

Receives BPDUs
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:

Discards frames received on the interface

Discards frames switched from another interface for forwarding

Does not learn addresses

Receives BPDUs
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:

Discards frames received on the interface

Discards frames switched from another interface for forwarding

Learns addresses

Receives BPDUs
Forwarding State
A Layer 2 interface in the forwarding state forwards frames. The interface enters the forwarding state from the learning
state.
An interface in the forwarding state performs these functions:

Receives and forwards frames received on the interface