3Com TR - Manual (Page 63)

Bridge Configuration 4-5
5 In the Spanning Tree field, select *802.1d* to enable
802.1d compliant Spanning Tree for the Switch 2000
TR. The BPDU value is automatically assigned:
80-01-43-00-00-00.
6 Select OK. Repeat for all ports as desired.
Spanning Tree Fields
Spanning Tree is an algorithm designed to eliminate
loops in networks. The algorithm detects and logically
prohibits redundant paths by placing some bridge
ports in the blocking state so that only one route exits
between any two LANs and therefore any two sta-
tions. After Spanning Tree eliminates redundant
paths, the network reconfigures and stabilizes. Any
time the network topology changes the algorithm
automatically re-learns the new configuration and
adjusts the bridge ports accordingly. This guarantees
that all stations are connected.
Spanning Tree Parameters
The following parameters are components of the
Spanning Tree algorithm. These are set to factory
defaults.
■ Priority—This setting influences the choice
between the root bridge and the designated
bridge. The bridge with the lowest priority is the
one most likely to be used as the root bridge by
Spanning Tree.
■ Entry Age—The period of time that must
expire between sends before a MAC address is
“aged out” (removed) from the switch data
base (transparent bridging table).
■
Bridge Hello Time—The period of time between
the generation of configuration messages by a
root bridge is known as Hello Time.
In networks where the probability of losing config-
uration messages is high, a short Hello Time is
desirable. In networks that are fairly stable, a
longer time lowers the overhead of the Spanning
Tree algorithm.
■ Bridge Max. Age—Maximum Age determines
the amount of time the current Spanning Tree con-
figuration data is judged too old and discarded
from the Switch 2000 TR’s memory. Maximum Age
should never be reached in a properly configured
Spanning Tree.
If this value is too small, the Spanning Tree must
configure too frequently. If this setting is too high
the network requires more time to adjust to
changes in the topology.
■ Forward Delay—The Forward Delay value deter-
mines the amount of time the Switch 2000 TR
spends in the “listening “and “learning” states.
This prevents the forwarding of data packets until
all parts of the topology have been notified of a
change in the topology.
A low value can result in temporary loops as the
Spanning Tree reconfigures the topology. A setting
that is too high forces the network to wait longer
for Spanning Tree to reconfigure the topology.
NOTE: For more information on Spanning Tree and
bridging concepts refer to Interconnections:Bridges
and Routers by Radia Perlman. Addison-Wesley Pub-
lishing Company, Reading MA. © 1992.