An asterisk in the output of the show ip pim tunnel command indicates that the router is a rendezvous point (RP). An RP is a well-connected, centrally located router that is responsible for keeping track of multicast group membership information. Protocol Independent Multicast sparse mode (PIMSM) requires an RP, whereas PIM dense mode(PIMDM) does not. When PIMSM is used, each multicast receiver must be able to reach the RP through a connected tree of PIMSM routers. If a router along the path is not configured for PIMSM, multicast receivers will not be able to register with the RP and multicast traffic will not flow to those receivers.

The following is sample output from the show ip pim tunnel command taken from a RP. The output is used to verify the PIM Encap and Decap Tunnel on the RP.
The asterisk (*) indicates that the router is the RP. The RP will always have a PIM Encap and Decap Tunnel interface.
Router# show ip pim tunnel Tunnel0 Type : PIM Encap RP : 192.168.2.6* Source: 192.168.2.6 Tunnel1 Type : PIM Decap RP : 192.168.2.6* Source: -

The following is sample output from the show ip pim tunnel command taken from a non-RP. The output is used to confirm that a PIM Encap Tunnel has been created on a non-RP router.
Router# show ip pim tunnel Tunnel0 Type : PIM Encap RP : 192.168.1.6 Source: 192.168.17.7