350-401 Question 204
Select 3A network engineer has configured multicast routing on a network using PIM-SM (Protocol Independent Multicast - Sparse Mode). During troubleshooting, they observe that multicast traffic is not being forwarded to a specific receiver. Upon investigation, they find that the Reverse Path Forwarding (RPF) check is failing. Which of the following could be causing the RPF check to fail?
- A
The multicast source is not reachable through the interface used by the unicast routing table.
- B
The multicast group address used is not registered with the Rendezvous Point (RP).
- C
A unicast routing loop exists in the network topology.
- D
The router does not have an entry for the multicast group in its IGMP membership table.
- E
The router is using a different incoming interface for the source traffic than what the unicast routing table indicates.
Show answer and explanation
Correct answers: A, C, E
Explanation
RPF checks are a critical part of multicast routing to ensure that multicast traffic is received on the correct interface, as determined by the unicast routing table. Failures can occur if the source is unreachable, a unicast routing loop exists, or the multicast traffic arrives on an unexpected interface. Understanding these failure scenarios helps ensure proper multicast traffic forwarding.
- A. Correct.
RPF checks verify that the multicast source is reachable via the interface that the unicast routing table specifies. If the source is not reachable, the RPF check will fail.
- B. Incorrect.
PIM-SM requires the multicast group to be registered with the Rendezvous Point (RP), but this is unrelated to RPF checks. The RPF check is focused on verifying the source's reachability.
- C. Correct.
A unicast routing loop can cause the RPF check to fail because the router might receive the multicast traffic on an unexpected interface, violating the RPF rule.
- D. Incorrect.
IGMP membership tables are used to track multicast group memberships on the last-hop router. A missing IGMP entry does not directly impact the RPF check.
- E. Correct.
An RPF check fails if the multicast traffic arrives on an interface that does not match the interface specified by the unicast routing table for reaching the source.