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352-001 · Question #557
352-001 Question #557: Real Exam Question with Answer & Explanation
The correct answer is B: It is switched due to a successful RPF check against the routing table. On a shared tree (*, G), the LHR performs an RPF check toward the RP using the unicast routing table, and if the packet arrives on the correct RPF interface, it is forwarded.
Question
Refer to the exhibit. As part of a redesign project, you must predict multicast behavior. What is the resultant multicast traffic receiving on the shared tree( , G), if it is received on the LHR interface indicated?
Exhibit
Options
- AIt is dropped due to an unsuccessful RPF check against the multicast receiver
- BIt is switched due to a successful RPF check against the routing table
- CIt is switched given that no RPF check is performed
- DIt is dropped due to an unsuccessful RPF check against the multicast source
Explanation
On a shared tree (*, G), the LHR performs an RPF check toward the RP using the unicast routing table, and if the packet arrives on the correct RPF interface, it is forwarded.
Common mistakes.
- A. RPF checks in multicast routing are never performed against the multicast receiver - receivers are downstream and do not factor into RPF logic.
- C. An RPF check is always performed on multicast traffic, including shared tree (*, G) entries; there is no scenario where multicast forwarding skips the RPF check entirely.
- D. Checking RPF against the multicast source applies to source-specific (S, G) entries, not shared tree (*, G) entries, where the RPF lookup targets the RP address instead.
Concept tested. PIM-SM shared tree RPF check behavior
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