Thisarticlecontinuesthediscussionofdesignchoicesforbuildingcost-effective,high-performanceBeowulfclusters.Afterselectingtheappropriatecomputenode(asdiscussedin“DesignChoicesforaCost-Effective,High-PerformanceBeowulfCluster,”Issue3,2000),thenextdesignstepistodecidethecommunicationssubsystem.