RPF Check Purposes in Multicast Routing
Short answer
AD 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.
Question
Key Takeaways
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.
Common mistake: 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.
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.
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?
Is RPF part of PIM or the underlying routing protocol?
Discussion 3 contributions
Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.
From public community threads 3
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cybman76 👍 3RPF checking destination address?????
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rickyarchi 👍 3A & C are correct
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e14efdb 👍 1RPF 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.
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