Routing data communications packets in a parallel computer that includes compute nodes organized for collective operations. Each compute node including an operating system kernel and a system-level messaging module that is a module of automated computing machinery that exposes a messaging interface to applications. Each compute node including a routing table that specifies, for each of a multiplicity of route identifiers, a data communications path through the compute node. Including to carry out the steps of: receiving in a compute node a data communications packet that includes a route identifier value; retrieving from the routing table a specification of a data communications path through the compute node; and routing, by the compute node, the data communications packet according to the data communications path identified by the compute node's routing table entry for the data communications packet's route identifier value.