3Com 530, 510, 520 manual Multiple Paths Between Bridged LANs, Figure A-2 Multiple Remote Bridges

Bridging and Routing Concepts A-5

Multiple Paths Between Bridged LANs

With only a single physical path between LANs, the network is susceptible to link and bridge failures. In the event of a failure, the connection between any of the LANs upstream or downstream of the point of failure will be broken. A more resilient network of interconnected LANs can be established by providing more than one link between any two of the LANs.

Normally, this network would soon encounter serious problems resulting from a loop, around which frames could endlessly travel if precautions aren’t taken by the bridges. Over ISDN links a proprietary form of loop control is implemented. On WAN leased line links the Spanning Tree Protocol (STP) prevents data loops.

On startup, the bridges send out frames to enquire if there are other bridges on the network. By exchanging information, the bridges block ports that cause the loops and ensure that there is only ever one active path through the network. If one of the links or bridges fail, the other bridges detect this and reconfigure their ports so that there is once again an active data path through the network.

Figure A-2 Multiple Remote Bridges