Allied Telesis AT-S86 - Manual (Page 98)

Chapter 10: STP and RSTP
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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 (BPDUs)
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-S86 management software. The interval is measured in seconds
and the default is two seconds. Consequently, if an AT-FS750/48 Fast
Ethernet Smart Switch is selected as the root bridge of a spanning tree
domain, it transmits a BPDU every two seconds.