Blocking State - Cisco 7609 Configuration Manual

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
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:
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Cisco 7600 Series Router Cisco IOS Software Configuration Guide—12.1E
15-8
15-3. After initialization, a BPDU is sent out to each Layer 2 LAN port. A network device
Segment
frames
Port 1
Station
addresses
BPDUs
Filtering System
database module
BPDUs
Data
frames
Port 2
Blocking
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.
Forwarding
Network
management
and data frames
Frame
forwarding
Network
management
frames
Segment
frames
Chapter 15 Configuring STP and IEEE 802.1s MST
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