3Com corebuilder 3500 Implementation Manual page 135

STP Bridge and Port Parameters 135
Bridge forward delay — The forward delay value specifies the
amount of time that a bridge spends in each of the listening and the
learning states. This value temporarily prevents a bridge from starting
to forward data packets to and from a link until news of a topology
change has spread to all parts of a bridged network. The delay gives
enough time to turn off to all links that need to be turned off in the
new topology before new links are turned on.
Setting the value too low can result in temporary loops while the
Spanning Tree algorithm reconfigures the topology. Setting the value
too high can lead to a longer wait while the STP reconfigures the
topology. The recommended value is 15 seconds.
The value that you set for bridge forward delay is only used if the
system is selected as the root bridge. Otherwise, the system uses the
value that is assigned to it by the root bridge.
STP group address — The STP group address is a single address to
which a bridge listens when it receives STP information. Each bridge
on the network sends STP packets to the group address. Every bridge
on the network receives STP packets that were sent to the group
address, regardless of which bridge sent the packets.
You may run separate STP domains in your network by configuring
different STP group addresses. A bridge only acts on STP frames that
are sent to the group address for which it is configured. Frames with a
different group address are ignored.
Because there is no industry standard about group address, bridges
from different vendors may respond to different group addresses. If
STP does not seem to be working in a mixed-vendor environment,
verify that all devices are configured with the same group address.