Allied Telesis AT-S39 - Manual (Page 85)

AT-S39 User’s Guide
85
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 temporarily 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-8024 and AT-
8024GB Fast Ethernet Switches 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 on-line, 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-8024 and AT-8024GB Fast Ethernet Switches. The
interval is measured in seconds and the default is 2 seconds.
Consequently, if an AT-8024 or an AT-8024GB Switch is selected as the
Root Bridge of a spanning tree domain, it will transmit a BPDU every two
seconds.