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Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
OL-8978-04
Chapter 7 Configuring Spanning Tree
Understanding How Spanning Tree Protocols Work
Figure 7-2 STP Port States
You can modify each port state by using management software, for example, 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.
Blocking State
A port in the blocking state does not participate in frame forwarding (see Figure 7-3). 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.
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