Blocking State - IBM BladeCenter Installation And User Manual

Note also that the example network topology is intended to provide redundancy to
protect the network against a link or port failure, not a switch failure or removal. For
example, a failure of switch A would isolate LAN 1 from connecting to LAN 2 or
LAN 3.

Blocking state

A port in the blocking state does not forward packets. When the switch is started, a
BPDU is sent to each port in the switch putting these ports into the blocking state. A
switch initially assumes it is the root, and then begins the exchange of BPDUs with
other switches. This will determine which switch in the network is the best choice
for the root switch. If there is only one switch on the network, no BPDU exchange
occurs, the forward delay timer expires, and the ports move to the listening state.
All STP enabled ports enter the blocking state following the switch startup.
A port in the blocking state does the following:
v Discards packets received from the network segment to which it is attached.
v Discards packets sent from another port on the switch for forwarding.
v Does not add addresses to its forwarding database
v Receives BPDUs and directs them to the central processing unit (CPU).
v Does not transmit BPDUs received from the CPU.
v Receives and responds to network management messages.
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Appendix D. Understanding and troubleshooting the Spanning Tree Protocol