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 thecontrol 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 reconvergence and on multipoint non-broadcast
interfaces.
PIM Dense Mode is different from SM in the sense that data-plane operations preclude controlplane signaling. One typical irresolvable RPF problem with PIM Dense mode is known as splithorizon 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-topoint links by the virtue of PIM NBMA mode.