Allied Telesis AT-S63 - Manual (Page 220)

Chapter 20: Spanning Tree and Rapid Spanning Tree Protocols
220 Section V: Spanning Tree Protocols
Forwarding Delay and 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 in the AT-S63 Management
Software. The appropriate value for this parameter depends on a number
of variables; the size of your network is 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.
Note
The forwarding delay parameter applies only to ports on the switch
that are operating STP-compatible mode.
Hello Time and
Bridge Protocol
Data Units
(BPDU)
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 or RSTP information. This portion of the frame is
referred to as the bridge protocol data unit (BPDU). When a bridge is
brought online, it issues 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 periodically transmits 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
in the AT-S63 Management Software. The interval is measured in