7-6
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
At any given time, each port on a switch using STP is in one of these states:
•
Blocking
•
Listening
•
Learning
•
Forwarding
•
Disabled
A port moves through these states:
•
From initialization to blocking
•
From blocking to either listening or disabled
•
From listening to either listening or disabled
•
From learning to either forwarding or disabled
•
From forwarding to disabled
Figure 7-2
illustrates how a port moves through the states.
Figure 7-2
STP Port States
You can modify each port state by using management software, such as the VLAN Trunking Protocol
(VTP). When you enable spanning tree, every switch in the network goes through the blocking state and
the transitory states of listening and learning at power up. If properly configured, each port stabilizes
into the forwarding or blocking state.
When the spanning tree algorithm places a port in the forwarding state, the following occurs:
•
The port is put into the listening state while it waits for protocol information that suggests it should
go to the blocking state
•
The port waits for the expiration of a protocol timer that moves the port to the learning state
•
In the learning state, the port continues to block frame forwarding as it learns station location
information for the forwarding database
•
The expiration of a protocol timer moves the port to the forwarding state, where both learning and
forwarding are enabled
Boot-up
initialization
Blocking
state
S5691
Listening
state
Disabled
state
Learning
state
Forwarding
state