Blocking State; Listening State; Learning State - Cisco NCS 4200 Series Configuration Manual

Blocking State

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
• 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
LAN Switching Configuration Guide, Cisco IOS XE Everest 16.6.1 (Cisco NCS 4200 Series)
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Configuring PVST+ and RPVST+