Blocking State; Listening State - Cisco WS-C2948G-GE-TX Configuration Manual

Chapter 7 Configuring Spanning Tree

Blocking State

A port in the blocking state, such as Port 2 in
initialization, a BPDU is sent to each port in the switch. A switch initially assumes that it is the root until
it exchanges BPDUs with other switches. This exchange establishes which switch in the network is
really the root. If only one switch resides in the network, no exchange occurs, the forward delay timer
expires, and the ports move to the listening state. A switch always enters the blocking state following
switch initialization.
Figure 7-3
A port in the blocking state performs as follows:
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•
•
•
•
•

Listening State

The listening state is the first transitional state that a port enters after the blocking state. The port enters
this state when the spanning tree determines that the port should participate in frame forwarding.
Learning is disabled in the listening state.
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
Port 2 in Blocking State
Segment
frames
Port 1
Station
addresses
BPDUs
Filtering System
database module
BPDUs
Data
frames
Port 2
Blocking
Discards frames that are received from the attached segment
Discards frames that are switched from another port for forwarding
Does not incorporate station location into its address database (there is no learning on a blocking
port, so there is no address database update)
Receives BPDUs and directs them to the system module
Does not transmit BPDUs that are received from the system module
Receives and responds to network management messages
Figure 7-3, does not participate in frame forwarding. After
Forwarding
Network
management
& data frames
Frame
forwarding
Network
management
frames
Segment
frames
Figure 7-4 shows a port in the listening state.
Understanding How STPs Work
7-7