Redundant topologies

A goal of redundant topologies is to eliminate network downtime caused by a single point of failure. All networks need redundancy for enhanced reliability. Network reliability is achieved through reliable equipment and network designs that are tolerant to failures and faults. Networks should be designed to reconverge rapidly so that the fault is bypassed.

In a redundant mesh topology network devices are connected with many redundant interconnections between network nodes. In a true mesh topology every node has a connection to every other node in the network. A mesh network with redundant connections is an example of a redundant topology. The level of redundancy can be measured by network connectivity.

On a redundantly connected network if a router were to fail connectivity would be preserved by routing traffic through a redundant connection. Furthermore, each router should have two or more points, or 'legs', with which to keep redundancy.

Failures within the Solution

Redundant network topologies are designed to ensure that networks continue to function in the presence of single points of failure. Work is interrupted less often for users because the network continues to function. Any interruptions that are caused by a failure should be as short as possible. Reliability is increased by redundancy. A network that is based on switches or bridges will introduce redundant links between those switches or bridges to overcome the failure of a single link. These connections introduce physical loops into the network. These bridging loops are created so if one link fails another can take over the function of forwarding traffic. In this transition, any unknown destination for currently flowing traffic is flooded over every port except the one it originated from. At layer 2, headers don't have a TTL (time to live) value. If a frame is sent into a layer 2 looped topology of switches, it would loop forever causing serious overhead on the bandwidth of the network.

Fix for the Failure

The ultimate solution is to allow physical loops, but create a loop free logical topology. This can be achieved by using the Spanning tree protocol. The STP applies a specific algorithm to the network based on its topology with the result of a loop free logical topology.

References