350-401 Question 202
Select 2A network engineer is implementing multicast routing in an enterprise network that uses Protocol Independent Multicast (PIM) in Sparse Mode. During troubleshooting, the engineer observes that multicast traffic is not being forwarded correctly. Upon investigation, they discover that the Reverse Path Forwarding (RPF) check is failing. What could be the possible reasons for the RPF check failure?
- A
The multicast routing table does not have a route to the source of the multicast traffic.
- B
A unicast routing table entry for the source points to a different interface than the one receiving the multicast traffic.
- C
The router does not have an IGMP membership report for the multicast group.
- D
The source of the multicast traffic is in a different autonomous system, and BGP is not configured to propagate multicast routes.
- E
The designated router (DR) has not been elected on the local segment.
Show answer and explanation
Correct answers: A, B
Explanation
The Reverse Path Forwarding (RPF) check is a key mechanism in multicast routing to ensure that multicast traffic flows along the correct path back to the source. For the RPF check to succeed, the router must have a valid route to the source in its unicast or multicast routing table, and the interface receiving the multicast traffic must match the interface used to reach the source. Failures in either of these conditions will cause the RPF check to fail, leading to dropped multicast packets.
- A. Correct.
Correct - The RPF check ensures that the router has a route to the source of the multicast traffic in its multicast routing table. If this route is missing, the RPF check will fail.
- B. Correct.
Correct - The RPF check verifies that the interface receiving the multicast traffic matches the interface used to reach the source in the unicast routing table. A mismatch will cause the RPF check to fail.
- C. Incorrect.
Incorrect - IGMP is used to manage group membership on a subnet but does not impact the RPF check, which focuses on source reachability and interface consistency.
- D. Incorrect.
Incorrect - While BGP can propagate multicast routes in certain scenarios, RPF check failures are not directly caused by the lack of BGP configuration across autonomous systems.
- E. Incorrect.
Incorrect - The election of a designated router (DR) is important for PIM operations on a segment but does not influence the RPF check.