3Com AccessBuilder 500, User Manual

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O U T I N G
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.