9E312/9E423 Module Bridging
3-12 Bridge Spanning Tree
In a network design with multiple transparent bridges placed in parallel (i.e.,
attached to the same local network segment), only a single bridge should forward
data through the LAN, leaving the remaining bridges on the segment in a standby
state so that another can assume the bridging responsibility (should the currently
active bridge go down). The Spanning Tree Algorithm (STA) is the method that
bridges use to communicate with each other to ensure that only a single data
route exists between any two end stations.
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 traffic and collisions that would cripple a network that
relied on carrier sense and collision detection). 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 Bridge Status window, click on . A pull-down menu will
appear.
2. Click on Spanning Tree.... The Bridge Spanning Tree window, Figure 3-4, will
appear.