Blocking State - Cisco Catalyst 2950 Software Manual

Understanding Basic STP Features
Figure 9-2
Figure 9-2
Power-on
initialization
Blocking
Listening
Learning
Forwarding
When you power up the switch, STP is enabled by default, and every interface in the switch, VLAN, or
network goes through the blocking state and the transitory states of listening and learning. Spanning tree
stabilizes each interface at the forwarding or blocking state.
When the spanning-tree algorithm places an interface in the forwarding state, this process occurs:
1.
2.
3.
4.

Blocking State

An interface in the blocking state does not participate in frame forwarding. After initialization, a BPDU
is sent to each interface in the switch. 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
interfaces move to the listening state. An interface always enters the blocking state following switch
initialization.
An interface in the blocking state performs as follows:
•
•
•
•
Catalyst 2950 Desktop Switch Software Configuration Guide
9-6
illustrates how an interface moves through the states.
Spanning Tree Interface States
state
Disabled
state
state
state
state
The interface is put in the listening state while spanning tree waits for protocol information to
transition the interface to the blocking state.
While spanning tree waits for 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 port
Discards frames switched from another interface for forwarding
Does not learn addresses
Receives BPDUs
Chapter 9 Configuring STP
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