Cisco Systems 6500, EA6500 Blocking State

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Catalyst 6500 Series Switch Cisco IOS Software Configuration Guide—Release 12.1 E
78-14099-04
Chapter 15 Configuring STP and IEEE 802.1s 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 15-3. 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 15-3 Interface 2 in Blocking State
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.
Filtering
database
Frame
forwarding
System
module
Port 1
BPDUs
Segment
frames
Segment
frames Forwarding
Blocking
BPDUs
Station
addresses
Network
management
and data frames
Port 2
S5692
Network
management
frames
Data
frames