Cisco Cisco Aironet 340 Ethernet Bridges Guia Do Utilizador

If the port is still part of the spanning tree at the end of the listening 
period it is put in the learning state. In this state it can still receive and 
transmit configuration messages, but is also allowed to learn the source 
addresses from the packets received from its LAN. It is still not allowed 
to forward any packets. The learning state is used to lessen the amount 
of flooding of unknown destination addresses that would occur if the 
port started forwarding before there were any entries in its learning 
table.

Once the learning period is over, the port is allowed to forward data 
normally.

The configured timeout values on the root bridge are passed to each 
bridge in a configuration message to ensure that all bridges on the infra-
structure are using the same timeout periods.

The root bridge puts its own values in its messages. All other bridges 
copy the values contained in the configuration message sent to them 
from their root port. The value in this message is used in all of the 
bridge’s transmitted messages. Using this method, the values are propa-
gated throughout the infrastructure.

Occasionally stations may be moved from one LAN to another. The 
bridges will remove learned addresses from their tables if no packets 
have been received from a node for a period of time. 

If node addresses do not timeout, the bridge may continue to send pack-
ets for a node to the wrong LAN. If a node sends packets from its new 
LAN location, the tables might be corrected, however, this is not guar-
anteed. The default timeout period is 5 minutes.

If a new bridge or port is added to an infrastructure, the ports included 
in the spanning tree could change dramatically. It may appear that a 
node has changed location very quickly.

To allow for these quick changes of location, the spanning tree protocol 
specifies that every time a port enters the blocked or forwarding states, 
its bridge must send a topology changed message to the root bridge.