Allied Telesis AT-8100 Series - Manual (Page 781)

AT-8100 Switch Command Line User’s Guide
781
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 in 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
on the switch. The interval is measured in seconds and has a default
setting of two seconds. Consequently, if the switch is selected as the root
bridge of a spanning tree domain, it transmits a BPDU every two seconds.