RPF Check Purposes in Multicast Routing

Design multicast services (SSM, PIM bidirectional, MSDP, service-reflection)

What are two purposes of the RPF check in multicast routing? (Choose two.)

to ensure that multicast packets are forwarded if they arrived on the interface used to route traffic back to the source address Correct Answer
to ensure that multicast packets are forwarded if they arrived on the interface used to route traffic to the destination address
to ensure that multicast packets are dropped if they arrived on the interface used to route traffic to the destination address
to ensure that multicast packets are dropped if they arrived on the interface used to route traffic back to the source address Correct Answer
to ensure that multicast packets, no matter the interface they arrived on are forwarded out all interfaces

Answer Correct answer: A, D — RPF ensures packets are forwarded only if they arrived on the interface used to route traffic back to the source address, and drops them if they arrived on any other interface.

Community Insight

Multicast questions often confuse RPF with destination-based forwarding; remember that RPF strictly checks the path back to the SOURCE, not the destination.

The Reverse Path Forwarding (RPF) check prevents multicast routing loops and ensures packets arrive via the correct interface. This page explains that RPF validates the source address against the unicast routing table to determine forwarding eligibility.

Candidates frequently select options involving the 'destination address' because standard unicast routing uses destination lookups. However, RPF is specifically designed to verify the return path to the source to prevent loops.

Community Discussion (3 comments)

e14efdb 👍 1 Selected: AD

RPF is checking the source interface, not the destination interface. in answer D is there something missing, traffic should be dropped if it did NOT arrive on its intended interface used to send back the traffic to the source.

rickyarchi 👍 3 Selected: AC

A & C are correct

cybman76 👍 3

RPF checking destination address?????

Comments & Corrections

No comments yet — spotted an error or have a note? Share it below.

Expert Analysis

Why the Answer Is Correct

The RPF (Reverse Path Forwarding) check serves two critical functions in multicast routing: loop prevention and ensuring reverse-path uniqueness. First, it ensures that multicast packets are forwarded ONLY if they arrive on the interface that the router would use to route traffic back to the source address (Option A). Second, by enforcing this rule, it inherently ensures that packets arriving on any other interface are dropped (Option D). If a packet arrives on an unexpected interface, the RPF check fails, and the packet is discarded.

Why the Other Options Are Wrong

Options B and C are incorrect because RPF does not check the destination address; it checks the source address. Multicast forwarding relies on the source tree, so verifying the path to the destination is irrelevant for RPF. Option E is incorrect because multicast is not flooded out all interfaces indiscriminately; it follows specific distribution trees (SPT or ASM) and requires a successful RPF check.

Community Comment Notes

Community consensus strongly supports the combination of A and D. One user noted that "RPF checking destination address?????" highlighting the common confusion between unicast and multicast lookup methods. Another user clarified that "traffic should be dropped if it did NOT arrive on its intended interface used to send back the traffic to the source," confirming that dropping invalid packets is the flip side of forwarding valid ones.

Official Reference

https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipmulticast/configuration/15-mt/ip-multicast-15-mt-book/ip-multicast-rpf.html

Exam Strategy

When answering multicast questions, always distinguish between the Source Address and Destination Address. Unicast routes look at the Destination; Multicast RPF looks at the Source. Memorize the phrase: 'RPF checks the path back to the Source.'

Frequently Asked Questions

Why does RPF check the source address instead of the destination?

Multicast routing builds distribution trees based on the source. Checking the path back to the source ensures the packet is coming from the correct direction, preventing loops.

Is RPF part of PIM or the underlying routing protocol?

RPF is a concept used by multicast protocols like PIM, but it relies on the underlying unicast routing table (OSPF, EIGRP, etc.) to perform the lookup.

More 300-420 FAQ →

Related Analysis

Practice All 300-420 Questions

Access 150 questions with complete answers and detailed explanations.

View Full 300-420 Practice Test →

← Back to 300-420 Study Guide