AT-S41 User’s Guide
82
The cost of a path is cumulative; the final cost of a path is the value of all
ports between a bridge and the root bridge.
If two paths have the same port cost, the preferred path is selected
through port priority. This is a value that you can adjust on a per port
basis on the switch.
Handling
Topology
Changes
If there is a change in the network topology due to a failure, removal, or
addition of any active components, the active topology also changes.
This may trigger a change in the state of some blocked ports. However, a
change in a port state is not activated immediately.
It might take time for the root bridge to notify all bridges that a topology
change has occurred, especially if it is a large network. If a topology
change is made before all bridges have been notified, a temporary data
loop could occur, and that could adversely impact network
performance.
To forestall the formation of temporary data loops during topology
changes, a port designated to change from blocking to forwarding
passes through two additional states, listening and learning, before it
begins to forward frames. The amount of time a port spends in these
states is set by the forwarding delay
value. This value states the amount
of time that a port spends in the listening and learning states prior to
changing to the forwarding state.
The forwarding delay value is adjustable on the AT-8326GB Fast Ethernet
switch through the management software. The appropriate value for
this parameter will depend on a number of variables, with the size of
your network being a primary factor. For large networks, you should
specify a value large enough to allow the root bridge sufficient time to
propagate a topology change throughout the entire network. For small
networks, you should not specify a value so large that a topology change
is unnecessarily delayed, which could result in the delay or loss of some
data packets.
Communicating
Between
Bridges
The bridges that are part of a spanning tree domain communicate with
each other using a bridge broadcast frame that contains a special
section devoted to carrying STP information. This portion of the frame is
referred to as the Bridge Packet Data Unit (BPDU). When a bridge is
brought online, it will issue a BPDU in order to determine whether a root
bridge has already been selected on the network and, if not, whether it
has the lowest bridge priority number of all the bridges and should
therefore become the root bridge.
