Cisco Cisco Aironet 340 Ethernet Bridges Guia Do Utilizador

All ports on a bridge, either the root port or the designated port for their 
LAN, are allowed to forward packets. All others are blocked and do not 
transmit or receive any data packets.

All root and blocked ports monitor the LANs to which they are con-
nected and watch for configuration messages transmitted by the desig-
nated bridge for the LAN. 

The STP protocol specifies a timeout period in which these ports must 
see at least one message. Each time a message is received, the timer is 
restarted. If the timeout period expires, the bridge assumes the desig-
nated bridge has failed.

The bridge will discard the saved value for the port, make the port the 
designated port for that LAN, and restart sending configuration mes-
sages. The bridge will also recalculate its values for the root bridge and 
root cost based on the active ports.

Other blocked ports on the same LAN will timeout and start to transmit 
messages. Eventually a new designated bridge, port, and root bridge will 
be determined.

It will take a non-zero amount of time for the protocol to determine a 
stable loop free topology due to the time for messages to pass from one 
end of the infrastructure to the other. If the ports were allowed to for-
ward while the protocol was stabilizing, then temporary loops could 
form. 

To avoid temporary loops, ports are not allowed to go immediately from 
the blocked state to the forwarding state. They must first go through a 
state called listening. In this state, they may receive and transmit config-
uration messages as needed but must block all data traffic. The time 
spent in the listening state must be at least twice the end-to-end transmit 
time of the infrastructure.