Cisco WS-X6K-SUP1A-2GE Software Guide - Page 118

Understanding How Spanning Tree Protocols Work Chapter 8 Configuring Spanning Tree At any given time, each port on a switch using a spanning tree protocol is in one of these states: • Blocking • Listening • Learning • Forwarding • Disabled A port moves through these states as follows: • From initialization to blocking • From blocking to listening or to disabled • From listening to learning or to disabled • From learning to forwarding or to disabled • From forwarding to disabled Figure 8-2 illustrates how a port moves through the states. Figure 8-2 STP Port States Boot-up initialization Blocking state Listening state Disabled state Learning state S5691 Forwarding state 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. Catalyst 6000 Family Software Configuration Guide-Releases 6.3 and 6.4 8-6 78-13315-02