Allied Telesis AT-S41 - Manual (Page 83)

AT-S41 User’s Guide
83
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-8350GB 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.
The root bridge will periodically transmit a BPDU to determine whether
there have been any changes to the network topology and to inform
other bridges of topology changes. The frequency with which the root
bridge sends out a BPDU is called the Hello Time. This is a value that you
can set on the AT-8350GB Fast Ethernet switches. The interval is
measured in seconds and the default is 2 seconds. Consequently, if an
AT-8350GB switch is selected as the root bridge of a spanning tree
domain, it will transmit a BPDU every 2 seconds.