Cisco 3845 Software Guide - Page 269

Chapter 13 Configuring STP Understanding Spanning-Tree Features Figure 13-1 shows how an interface moves through the states. Figure 13-1 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 port on the switch, as well as any other port in other switches in the VLAN or network that are participating in spanning tree, 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 spanning-tree 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. Blocking State A Layer 2 interface in the blocking state does not participate in frame forwarding. After initialization, a BPDU is sent to each switch interface, or to each switch STP port. A switch initially functions as the root until it exchanges BPDUs with other switches. This exchange establishes which switch in the network is the root or root switch. If there is only one switch in the network, no exchange occurs, the forward-delay timer expires, and the interface moves to the listening state. An interface participating in spanning tree always enters the blocking state after switch initialization. An interface in the blocking state performs these functions: • Discards frames received on the interface • Discards frames switched from another interface for forwarding • Does not learn addresses • Receives BPDUs OL-23400-01 Cisco ME 3800X and 3600X Switch Software Configuration Guide 13-5