Allied Telesis AT-S62 - Manual (Page 335)

AT-S62 User’s Guide
Section IV: Spanning Tree Protocols 335
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-S62 management
software. The appropriate value for this parameter depends on a
number of variables, 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 network traffic.
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-S62 software. The interval is measured in seconds and
the default is two seconds. Consequently, if an AT-8524M switch is
selected as the root bridge of a spanning tree domain, it transmits a
BPDU every two seconds.