Cisco 2950 Software Configuration Guide - Page 299

Chapter 14 Configuring STP Understanding Spanning-Tree Features • Forwarding-The interface forwards frames. • Disabled-The interface is not participating in spanning tree because of a shutdown port, no link on the port, or no spanning-tree instance running on the port. An interface moves through these states: • 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 14-1 illustrates how an interface moves through the states. Figure 14-1 Spanning-Tree Interface States Power-on initialization Blocking state Listening state Disabled state Learning state 43569 Forwarding state When you power up the switch, spanning tree is enabled by default, and every interface in the switch, VLAN, or network goes through the blocking state and the transitory states of listening and learning. Spanning tree stabilizes each interface at the forwarding or blocking state. When the spanning-tree algorithm places a Layer 2 interface in the forwarding state, this process occurs: 1. The interface is in the listening state while spanning tree waits for protocol information to transition the interface to the blocking state. 2. While spanning tree waits the forward-delay timer to expire, it moves the interface to the learning state and resets the forward-delay timer. 3. In the learning state, the interface continues to block frame forwarding as the switch learns end-station location information for the forwarding database. 4. When the forward-delay timer expires, spanning tree moves the interface to the forwarding state, where both learning and frame forwarding are enabled. 78-11380-10 Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide 14-5