Which two steps does a router take to do an RPF check? (Choose two.)

Question

Options

• AIf the packet has not arrived on the interface leading back to the source, the packet is dropped.
• BThe router looks up the destination address in the unicast routing table to determine if the packet
• CIf the packet has not arrived on the interface leading back to the source, the packet is forwarded.
• DThe router looks up the source address in the unicast routing table to determine if the packet has
• EIf the packet has arrived on the interface leading back to the source, the packet is dropped.

• A
  91% (59)
• B
  5% (3)
• C
  2% (1)
• E
  3% (2)

Explanation

Reverse Path Forwarding (RPF) is a multicast security and loop-prevention mechanism with two key steps: Step 1 (Answer D) - The router looks up the SOURCE address of the incoming multicast packet in its unicast routing table to identify which interface is the best path back toward that source (the RPF interface). Step 2 (Answer A) - The router checks whether the multicast packet arrived on that RPF interface. If the packet did NOT arrive on the interface that leads back to the source, the packet is dropped, as it likely arrived via a suboptimal or loop-creating path. If it DID arrive on the RPF interface, the packet is forwarded. Option B is wrong because RPF checks the source address, not the destination. Option C is wrong because a failed RPF check results in a drop, not a forward. Option E is wrong because arriving on the correct RPF interface means the packet is forwarded, not dropped.

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