Redundancy, Loop Avoidance and Spanning Tree Protocol (STP) Mode

Because of the considerable negative effects that loops can have upon networks and network performance it is imperative that we keep a really close eye on the management and control of those that do exist for reasons of redundancy and all round network robustness and resilience.

Redundancy and Loops

Whenever a redundant pathway is added to a network a loop is created. The greater the number of redundant pathways that exist in a network the more loops that will be created. This also means that the greater the number of loops in a network the greater the combined crippling effect they will have. Note that loops will come into being whenever there are two or more transparent bridges/switches on the same network segment.

Plug ‘n’ Play (unmanaged) Devices

Loops are particularly difficult to deal with whenever unmanaged Plug ‘n’ Play devices such as transparent bridging devices are involved because they do not have a user accessible interface. The implications of this are that the device must somehow manage the situation itself.

Redundancy, Reliability and Robustness

Introducing alternative pathways by which a network segment or device can be reached (even if this means taking the scenic route) adds significantly to the overall reliability and robustness of a network. There are a number of other factors which on the one hand are highly desirable features for a network to have but also contribute to the creation of networking loops include:

Redundant fail-over solutions to minimize network and systems outages and downtime , multiple hot-swappable devices to act as a “safety-net”, duplicate fully powered-up “spare” mission critical systems ready to spring into action should the primary mission critical applications and devices become compromised or fail.

Switch Ports

When devices are reachable via more than one switch port the switch will use the spanning tree algorithm to determine the lowest “cost” path to said device(s)/segment(s) and then selectively shut down the other ports which had a higher “cost” value.

Spanning Tree Protocol (STP) Mode

When network infrastructure devices such as transparent bridging devices, work group switches and most managed switches etc are placed into Spanning Tree Protocol (STP) Mode they use the spanning tree algorithm to determine the “lowest cost path” to other devices and in so doing counteract the negative effects and consequences of loops and redundant pathways.

The reason for this is that devices in STP Mode will detect and automatically shut down all alternative redundant pathways. It is in this way that Spanning Tree Protocol Mode devices implement loop avoidance.

Thus administrators can introduce redundant pathways and redundant switches into their networks without creating loop-hell as redundant links are selectively shut down and remain in the shut down state for as long as the primary (lowest cost path) path remains up.

Should the lowest cost path/port fail or become unavailable the STP Mode device will immediately use the spanning tree algorithm to recalculate the network and the redundant pathway port will become active. The message will still get through.