USC Cisco Cisco Cisco Cisco USC This specification defines a multicast routing algorithm efficient for multicast groups that are densely distributed across a network. This protocol does not have a topology discovery mechanism often used by a unicast routing protocol. It employes the same packet formats sparse- mode PIM [PIMSM] uses. This protocol is called dense-mode PIM. The foundation of this design was largely built on Deering's early work on IP multicast routing [Deering91].