We investigate techniques for the support of multicast traffic in IP routers that have a large number of switch fabric ports (from 128 to 1024) and internally operate on relatively small fixed-size data units (cells of 64 bytes each). In small packet switches, multicast traffic is typically handled by prepending a local multicast label (LML) to each cell. The LML consists of a bitmap with as many bits as switch ports. The bitmap identifies the set of ports to which copies of the cell must be transferred. The bitmap approach is no longer feasible in switches with 128 ports or more, where the LML length cannot be smaller than 16 bytes, an intolerable overhead when the internal cell payload is only 64 bytes. We devise several compression algorithms, both static and adaptive, to reduce the size of the LML's to be attached to multicast cells. The algorithms define compressed representations of the distribution sets of the multicast cells, trading the additional bandwidth needed to transfer redundant cell copies (i.e., copies directed to outputs that do not belong to the actual distribution sets) for the cell header overhead otherwise needed to deliver the cells only to the proper outputs. We use simulation experiments to compare the performance of the compression algorithms.