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 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.
Network Topology
If your network topology is star shaped, a combination of ISDN and bridging is usually the most efficient and successful option. Routing is a better solution if your network topology is a complex mix of both leased line and ISDN circuits, running at 64 Kbps to 2 Mbps.
Broadcast Storms
Bridges are programmed to automatically forward data packets by default while routers filter data packets by default. These attributes have an impact on the overall flow of data across the network. Much has been made of broadcast storms in connection with bridged networks, where the broadcast signals from bridges propagate to fill all of the wide area bandwidth, and bring the network down. Broadcast storms cannot be attributed to installation of bridges or routers, but by poor protocol implementation and network design. However the deployment of routers can effectively firewall one logical network from another.
