An approach to building a large ATM switch is to simply set up a regularly-structured network in which smaller switch modules are interconnected. Routing is an issue if there are multiple paths from any input to any output in such a network. We focus on the 3-stage Clos network, not only because it is the architecture of choice for several potential switch manufacturers, but also because its high connectivity poses a stringent test on routing algorithms. One optimal and two heuristic algorithms have been designed and tested. Our results show that the heuristic algorithms can find multicast routes that are close to optimal within a response time that is significantly lower than that of the optimal algorithm. Further analysis of the experimental data suggests a hybrid implementation in which the optimal and heuristic algorithms are run in parallel with a set time limit. Finally, although this paper is motivated by the Clos switching network, the algorithms and the discussion here also apply to communications networks with a two-hop structure.
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