350-401 Question 203
Single answerA network engineer is troubleshooting a multicast routing issue in a campus network running PIM-SM (Protocol Independent Multicast - Sparse Mode). Hosts in one subnet report that they are unable to join a multicast group. The engineer verifies that the multicast group is active and that the appropriate traffic is being sent by the source. After inspecting the router's configuration, the engineer notices that the Reverse Path Forwarding (RPF) check is failing for the multicast traffic. What is the most likely reason for the RPF check failure?
- A
The multicast source's route is missing from the unicast routing table.
- B
The IGMP snooping feature is disabled on the switches.
- C
The rendezvous point (RP) is not configured for the multicast group.
- D
The multicast traffic is being forwarded through a non-designated router.
Show answer and explanation
Correct answer: A
Explanation
The RPF check ensures multicast traffic is received on the interface that aligns with the shortest path to the source, as determined by the unicast routing table. If the multicast source's route is missing from the unicast routing table, the router cannot verify the correct incoming interface, causing the RPF check to fail and the multicast traffic to be dropped.
- A. Correct.
The RPF check relies on the unicast routing table to determine the correct interface and path to the multicast source. If the source's route is missing, the RPF check will fail, causing multicast traffic to be dropped.
- B. Incorrect.
IGMP snooping is used by switches to limit multicast traffic to only interested receivers. While disabling IGMP snooping could cause unnecessary flooding, it does not directly cause RPF check failures.
- C. Incorrect.
The RP is necessary for building multicast distribution trees in PIM-SM. However, the absence of an RP does not lead to RPF check failures, as RPF is based on unicast routing rather than RP configuration.
- D. Incorrect.
The designated router (DR) is responsible for forwarding multicast traffic to the RP in a PIM-SM environment. However, traffic originating from a source does not rely on the DR for RPF checks, so this would not cause an RPF check failure.