3com TR User Manual download pdf (Page 63)

Bridge Conﬁguration 4-5

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-

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