Cabletron Systems NB20E Primary and Redundant Data Paths, 2 Multiple Bridging Environments

INTRODUCTION

1.5.2Primary and Redundant Data Paths

To ensure the integrity of the network, primary data paths through designated bridges, and redundant data paths through alternative bridges are determined in each LAN or loop configuration in the network. A LAN segment’s designated bridge is the bridge in a LAN or loop configuration with the lowest cost path of transmission from that LAN segment to the root bridge. The lowest cost path can be translated in a general way to mean the data transmission path to the root that is most economical.

The bridges in the network send out BPDUs to the other bridges so that they can all learn which is the bridge in the network, or in a potential loop configuration, with the lowest cost path to the root. The bridges can then determine their own priorities and roles as designated or redundant bridges. Redundant bridges are “blocked” from forwarding packets, but they continue to receive topology information.

Figure 1-2 provides a sample illustration of how a potential data loop is prevented in a simple multibridge environment. A data loop exists

NODE A

T

T

BRIDGE 1

NB20E OR 25E

T

T

NB20E OR 25E

BRIDGE 2

NB20E OR 25E

BRIDGE 3

T

T

NODE B

Figure 1-2 Multiple Bridging Environments

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