Allied Telesis AT-S60 - Manual (Page 191)

Chapter 12: STP, RSTP, and MSTP
Section II: Local and Telnet Management 190
The forwarding delay value is adjustable on the AT-8400 Series switch
through the management software. The appropriate value for this
parameter depends 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.
Note
The forwarding delay parameter applies only to STP.
Hello Time and Bridge Packet Data Units (BPDU)
The bridges in a spanning tree domain communicate with each other
using a bridge multicast frame that contains a special section devoted to
carrying STP or RSTP information. This portion of the frame is referred to
as the Bridge Packet Data Unit (BPDU). When a bridge is brought on-line,
it issues a BPDU in order to determine whether a root bridge has already
been selected on the network. If a root bridge has not been selected, the
BPDU determines whether it has the lowest bridge priority number of all
the bridges and, consequently, should 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 AT-8400 Series switch. The interval is measured in seconds
and the default is 2 seconds. Consequently, if an AT-8400 Series switch is
selected as the Root Bridge of a spanning tree domain, it transmits a
BPDU every two seconds.
Point-to-Point Ports and Edge Ports
Note
This section applies only to RSTP.
Part of the task of configuring RSTP is defining the port types on the
bridge. This relates to the device(s) connected to the port. With port type
defined, RSTP can reconfigure a network much quicker than STP when a
change in network topology is detected.