300-420 Question 281
Select 3An enterprise network has implemented multicast to efficiently deliver live video streams to multiple branch offices. The design uses Protocol Independent Multicast (PIM) in Sparse Mode (PIM-SM). During troubleshooting, it is observed that multicast traffic is not reaching some branches. Upon investigation, it is found that the issue relates to the Reverse Path Forwarding (RPF) check. Which of the following actions can resolve this issue?
- A
Ensure that the unicast routing table has the correct route to the multicast source.
- B
Configure a static multicast route to override the RPF check.
- C
Enable bidirectional PIM on all routers in the multicast domain.
- D
Verify that the Rendezvous Point (RP) is reachable by all routers in the multicast domain.
- E
Switch from PIM-SM to PIM Dense Mode (PIM-DM) to bypass RPF checks.
Show answer and explanation
Correct answers: A, B, D
Explanation
The Reverse Path Forwarding (RPF) check is a critical mechanism in multicast routing to ensure that multicast traffic is arriving on the correct interface. Failures in the RPF check often occur due to issues in the unicast routing table, which can be resolved by ensuring correct routes or configuring static multicast routes. Additionally, the Rendezvous Point (RP) must be reachable to ensure the proper construction of the multicast distribution tree. Understanding and addressing these factors can resolve RPF issues in PIM-SM networks.
- A. Correct.
Correct. The RPF check relies on the unicast routing table to determine the expected interface for multicast traffic. If the unicast route to the source is incorrect or missing, the RPF check will fail.
- B. Correct.
Correct. Static multicast routes can be configured to override the RPF check for specific multicast groups or sources, which can resolve the issue when no valid unicast route exists.
- C. Incorrect.
Incorrect. Bidirectional PIM is not directly related to resolving RPF check issues; it is used to optimize certain types of multicast traffic, such as many-to-many communication.
- D. Correct.
Correct. The Rendezvous Point (RP) must be reachable by all routers in the multicast domain during the initial multicast tree setup. An unreachable RP can cause RPF checks to fail.
- E. Incorrect.
Incorrect. Switching to PIM Dense Mode does not address issues with RPF checks; it uses a flood-and-prune mechanism, which is less efficient and not recommended in sparse multicast environments.