Cisco WS-SUP32-GE-3B Software Configuration Guide - Page 231

Chapter 17 Configuring STP and MST Understanding How STP Works Blocking State A Layer 2 LAN port in the blocking state does not participate in frame forwarding, as shown in Figure 17-3. After initialization, a BPDU is sent out to each Layer 2 LAN port. A network device initially assumes it is the root until it exchanges BPDUs with other network devices. This exchange establishes which network device in the network is the root or root bridge. If only one network device is in the network, no exchange occurs, the forward delay timer expires, and the ports move to the listening state. A port always enters the blocking state following initialization. Figure 17-3 Interface 2 in Blocking State Segment frames Forwarding Station addresses Port 1 BPDUs Network management and data frames Filtering database System module Frame forwarding S5692 BPDUs Data frames Port 2 Network management frames Blocking Segment frames A Layer 2 LAN 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 end station location into its address database. (There is no learning on a blocking Layer 2 LAN 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 Layer 2 LAN port enters after the blocking state. The Layer 2 LAN port enters this state when STP determines that the Layer 2 LAN port should participate in frame forwarding. Figure 17-4 shows a Layer 2 LAN port in the listening state. OL-11439-03 Catalyst Supervisor Engine 32 PISA Cisco IOS Software Configuration Guide, Release 12.2ZY 17-7