In PIM-SM what control plane signaling must a multicast source perform before it begins to send multicast traffic to a group?
A.
The source must send a PIM Register message to the rendezvous point (RP).
B.
The source must first join the multicast group using IGMP before sending.
C.
The source must perform a Request to Send (RTS) and Clear to Send (CTS) handshake with the PIM designated router (DR).
D.
No control plane signaling needs to be performed; the source can simply begin sending on the local subnet.
Explanation:
The most common type of multicast issue is the RPF Failure. RPF checks are used both at the control and data plane of multicast routing. Control plane involves PIM signaling some PIM messages are subject to RPF checks. For example, PIM (*,G) Joins are sent toward the shortest path to RP. Next, the BSR/RP address in the BSR messages is subject to RPF check as well. Notice that this logic does not apply to PIM Register messages the unicast register packet may arrive on any interface. However, RPF check is performed on the encapsulated multicast source to construct the SPT toward the multicast source.
Data plane RPF checks are performed every time a multicast data packet is received for forwarding. The source IP address in the packet should be reachable via the receiving interface, or the packet is going to be dropped. Theoretically, with PIM Sparse-Mode RPF checks at the control plane level should preclude and eliminate the data-plane RPF failures, but data-plane RPF failures are common during the moments of IGP re-convergence and on multipoint non-broadcast interfaces.
PIM Dense Mode is different from SM in the sense that data-plane operations preclude control-plane signaling. One typical irresolvable RPF problem with PIM Dense mode is known as split-horizon forwarding, where packet received on one interface, should be forwarded back out of the same interface in the hub-and-spoke topology. The same problem may occur with PIM Sparse mode, but this type of signaling allows for treating the NBMA interface as a collection of point-to-point links by the virtue of PIM NBMA mode.