Performance of the rapid spanning tree protocol in ... - RuggedCom

4.2 Root Bridge FailureUnfortunately, the additional complexity of meshed topology causes root bridge failure failover times toincrease compared with the simple ring. The problem of the root bridge failure scenario is that, after thefailure, every switch holding the obsolete Root information is feeding it back to the network, thus“confusing” all other switches and making them “reject” the right new Root information. The sameproblem exists in the ring topology as well but, due to the topology simplicity, there are no loops wherethe obsolete information would circulate and the “confusion” is fixed very quickly. This is not the case inmeshed networks.A meshed network can be looked at as multiple smaller rings interconnected with each other at multiplepoints. This topology allows the obsolete Root information to circulate multiple times back and forthinside and between the smaller rings. The network will not be confused forever - according to the RSTPstandard, the old information will be aged out after it traverses the maximum allowed number of hops(normally 20). However, the network outage time can last as long as seconds. It is caused by the fact thatmultiple switches in the network are continuously busy with processing and forwarding contradictoryBPDUs being continuously received on different ports.Root bridge failure in a meshed network is very hard to analyze and predict, and a result may be totallydifferent for every specific topology. The common conclusion is though, that the root bridge failover timegrows exponentially as more redundant paths are added to the network topology. The root bridge failurein meshed topology is a well recognized problem. Although some proprietary mechanisms are offered toindirectly improve the RSTP performance, no common solution is known for this case. This is the pricepaid for the high level of link redundancy provided by meshed network topology.17