Cisco WS-C2980G-A Software Guide - Page 103

Chapter 7 Configuring Spanning Tree Understanding How STPs Work Blocking State A port in the blocking state, such as Port 2 in Figure 7-3, does not participate in frame forwarding. After initialization, a BPDU is sent to each port in the switch. A switch initially assumes 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. Figure 7-3 Port 2 in Blocking State Segment frames Forwarding Station addresses Port 1 BPDUs Network management & data frames Filtering database System module Frame forwarding S5692 BPDUs Data frames Port 2 Network management frames Blocking Segment frames A port in the blocking state performs as follows: • Discards frames received from the attached segment • Discards frames switched from another port for forwarding • Does not incorporate station location into its address database (there is no learning on a blocking port, so there is no address database update) • Receives BPDUs and directs them to the system module • Does not transmit BPDUs received from the system module • Receives and responds to network management messages Listening State The listening state is the first transitional state a port enters after the blocking state. The port enters this state when the spanning tree determines that the port should participate in frame forwarding. Learning is disabled in the listening state. Figure 7-4 shows a port in the listening state. 78-15486-01 Catalyst 4500 Series, Catalyst 2948G, Catalyst 2980G Switches Software Configuration Guide-Release 8.1 7-7