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NB-30 Bridging
3-18 Bridge Spanning Tree
In Transparent bridging, Spanning Tree must be used to prevent data loops (since
in an Ethernet environment, a packet propagated down multiple paths would
cause higher volumes of trafÞc and collisions that would cripple a network that
relied on carrier sense and collision detection).
In a network designed with multiple bridges placed in parallel, Spanning Tree
selects a controlling Root Bridge and Port for the entire bridged local area
network, and a Designated Bridge and Port for each individual network segment.
The Root bridge is the one that selects one of two or more available bridge paths
between two end stations, basing its decision on factors associated with each of
the bridges in the path. A Designated Port/Bridge for a network segment relays
frames toward the Root Bridge, or from the Root Bridge onto the network
segment. When data passes from one end station to another across a bridged local
area network, it is forwarded through the Designated Bridge/Port for each
network segment towards the Root Bridge, which in turn forwards frames
towards Designated Bridges/Ports on its opposite side.
During the Root Bridge selection process, all bridges on the network
communicate STA information via Bridge Protocol Data Units (BPDUs). It is with
BPDUs that the bridges collectively determine the current network topology and
ensure that all bridges have current topology information.
To access the Bridge Spanning Tree window:
1. From the NB-30 Bridge Status window, click on Bridge. The Bridge menu will
appear.
2. Click on Spanning Tree.... The Bridge Spanning Tree window, Figure 3-6, will
appear.
Figure 3-6. Bridge Spanning Tree Window