You are auditing the network design of a PIM BiDir multicast network. Which option is the most appropriate multicast group source discovery design consideration for this design?

A phantom RP is used in PIM BiDir for RP address redundancy and high availability, not for advertising or discovering multicast group sources.

In PIM BiDir, all traffic flows bidirectionally on a shared tree between sources and receivers via the Rendezvous Point. Sources simply forward packets toward the RP address and the RP distributes them downstream on the shared tree, with no source registration, no SPT switchover, and no source discovery mechanism of any kind - the concept is architecturally absent from BiDir.

DNS is not a standard multicast source discovery mechanism in any PIM deployment model.

Statically configuring subscribers with specific sources is a design approach for PIM-SSM (Source-Specific Multicast), not a relevant consideration for PIM BiDir.

MSDP (Multicast Source Discovery Protocol) is used for inter-domain source discovery between PIM-SM sparse mode networks and has no role in PIM BiDir deployments.