7-7
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
Chapter 7 Configuring Spanning Tree
Understanding How STPs Work
Blocking State
A port in the blocking state, such as Port 2 in
Figure 7-3
, does not participate in frame forwarding. After
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
Port 2 in Blocking State
A port in the blocking state performs as follows:
•
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
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.
Figure 7-4
shows a port in the listening state.
Filtering
database
Frame
forwarding
System
module
Port 1
BPDUs
Segment
frames
Segment
frames
Forwarding
Blocking
BPDUs
Station
addresses
Network
management
& data frames
Port 2
S5692
Network
management
frames
Data
frames