350-401 Question 201
Select 3A network engineer is configuring multicast routing in a network. They observe that multicast traffic is not reaching the intended receivers. Upon investigation, they find that Reverse Path Forwarding (RPF) failures are occurring. Which of the following actions can help resolve the issue?
- A
Ensure that the multicast source traffic is routed through the same interface as the RPF check.
- B
Configure a static multicast route to override the RPF check.
- C
Enable IGMP snooping on all Layer 2 switches in the path.
- D
Verify that the unicast routing table has the correct source address entry.
- E
Switch from PIM Sparse Mode to PIM Dense Mode.
Show answer and explanation
Correct answers: A, B, D
Explanation
Reverse Path Forwarding (RPF) is a fundamental check performed by multicast routers to ensure packets are arriving on the correct interface based on the source IP address. RPF failures occur when this check fails, often due to misaligned unicast routing or incorrect interface usage. Ensuring proper unicast routing, configuring static multicast routes if necessary, and verifying the multicast path alignment with the RPF check are effective ways to resolve such issues. IGMP snooping and switching PIM modes do not directly address RPF failures.
- A. Correct.
Ensuring that the multicast source traffic is routed through the same interface as the RPF check is necessary for the RPF check to succeed. If the traffic arrives on a different interface, the router will drop the packets.
- B. Correct.
A static multicast route can be configured to override the RPF check, allowing the traffic to pass even if the default unicast routing table does not align with the multicast path.
- C. Incorrect.
Enabling IGMP snooping on Layer 2 switches improves multicast efficiency by limiting traffic to only relevant ports, but it does not directly address RPF failures.
- D. Correct.
The RPF check uses the unicast routing table to determine the best path back to the source. If the unicast routing table is incorrect, it can cause RPF failures, so verifying the table is critical.
- E. Incorrect.
Switching to PIM Dense Mode changes how multicast traffic is forwarded but does not resolve RPF failures, as they pertain to the underlying unicast routing and RPF logic.