Refer to the exhibit. As part of a redesign project, you must predict multicast behavior. What happens to the multicast traffic received on the shared tree (*, G), if it is received on the LHR interface indicated?
A.
It is switched due to a successful RPF check against the routing table.
B.
It is switched given that no RPF check is performed.
C.
It is dropped due to an unsuccessful RPF check against the multicast receiver.
D.
It is dropped due to an unsuccessful RPF check against the multicast source.
I think D should be the answer...
Behavior of Shared Tree ((*,G)) in PIM-SM:
1. The receiver sends an IGMP join.
2. The LHR creates a (*,G) entry and sends a PIM join toward the RP.
3. The multicast source registers with the RP via the FHR.
4. Multicast traffic starts arriving directly at the LHR from the FHR (not from the RP).
Now the issue arises:
- The (*,G) entry on the LHR expects traffic from the RP (via the RPF interface toward the RP).
- But traffic arrives from a different direction — directly from the source (via the FHR).
Result:
A Reverse Path Forwarding (RPF) check is performed on incoming multicast traffic.
- For (*,G) entries, the RPF check is against the RP.
- Since the traffic is not arriving on the RPF interface toward the RP, the RPF check fails.
Thus, the traffic is dropped.
"When multicast traffic arrives at a router on an interface that is not the RPF interface (toward the RP for (*,G)), the traffic is dropped."
— Cisco IP Multicast Design Guide
— PIM Sparse Mode Forwarding Behavior
In PIM Sparse Mode, multicast traffic received for a (*,G) entry must arrive on the RPF interface toward the RP. If it doesn’t, it will be dropped.
Trick here is, for (*,G) entry, RPF interface is one towards the RP & hence, in this case RPF check will be a success. BTW this is a normal operation of MCAST, without this it would never initiate a SPT switchover.
(*,G) reflects default group forwarding:
– IIF = RPF interface toward RP
https://yurmagccie.wordpress.com/2015/09/17/multicast-part-4-pim-sparse-mode/
Good link: https://www.juniper.net/documentation/en_US/junos-space-apps/connectivity-services-director5.0/topics/concept/multicast-rendezvous-points-shared-trees.html
A. https://www.cisco.com/c/en/us/support/docs/ip/ip-multicast/16450-mcastguide0.html
When a multicast packet arrives on an interface, the RPF process checks to ensure that this incoming interface is the outgoing interface used by unicast routing in order to reach the source of the multicast packet. This RPF check process prevents loops. Multicast routing does not forward a packet unless the source of the packet passes a RPF check. Once a packet passes this RPF check, multicast routing forwards the packet based only upon the destination address.
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