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Multicast routing protocols and routers on the Internet enable multicast transmission by replicating packets close to the destinations, obviating the need for multiple unicast connections, thereby saving network bandwidth and improving throughput. Similarly, within a router, multicast between linecards is enabled by a multicast capable switch fabric. A multicast cell is sent once from the source linecard to the switch fabric; the switch fabric sends the cells to all the destination linecards obviating the need for, and the waste of, linecard to fabric bandwidth that would result from multiple unicast cell transmissions. For high capacity routers (several terabits), the fixed size destination field of the cell is inadequate to specify exactly the subset of the switch ports the multicast cell should be sent to the number of multicast connections to be supported. Therefore, for several connections, we have to supercast, i.e., send the cell to non-subscribing linecards and have them drop the cell. We study the problem of assigning destination labels for multicast cells so that the amount of supercast, i.e., wasted bandwidth, is minimized, and the throughput of the router is maximized. We formalize this combinatorial optimization problem and prove it NP-complete and hard to find approximate solutions. We have devised several heuristic algorithms that we have implemented and we report the experimental results. Faster heuristics can support a higher multicast connection establishment rate; slower heuristics can be invoked off-line to further optimize multicast label maps.