Allied Telesis AT-S25 - Manual (Page 97)

AT-S25 Management Software User’s Guide
97
To forestall the formation of temporarily 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.
During a reconvergence and for a short period of time thereafter, the
switch’s FDB aging time is shortened considerably. This is done to
minimize the impact of possible path changes to host machines due to
the topology change. The side effect of this is an increase in flooding,
and may result in a small amount of packet loss.
The forwarding delay value is adjustable on the AT-8316F or AT-8324
Switch through the management software. The appropriate value for
this parameter will depend 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.
Hello Time and Bridge Packet 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 Packet Data Unit (BPDU). When a
bridge is brought on-line, it will issue 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 will periodically transmit 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
could set on the AT-8316F or AT-8324 Switch. The interval is measured in
seconds and the default is 2 seconds. Consequently, if an AT-8316F or an
AT-8324 Switch is selected as the Root Bridge of a spanning tree domain,
it will transmit a BPDU every two seconds.