300-420 · Question #315
What is the purpose of the RPF check in multicast routing?
The correct answer is A. to ensure that multicast packets are forwarded if they arrived on the interface used to route traffic. The Reverse Path Forwarding (RPF) check in multicast routing is designed to prevent routing loops and ensure that multicast packets are forwarded only if they arrive on the correct interface, which is the interface leading back to the source.
Question
What is the purpose of the RPF check in multicast routing?
Options
- Ato ensure that multicast packets are forwarded if they arrived on the interface used to route traffic
- Bto ensure that multicast packets are dropped if they arrived on the interface used to route traffic to
- Cto ensure that multicast packets are forwarded if they arrived on the interface used to route traffic
- Dto ensure that multicast packets are dropped if they arrived on the interface used to route traffic
- Eto ensure that multicast packets, no matter the interface they arrived on, are forwarded out all
How the community answered
(41 responses)- A90% (37)
- B5% (2)
- C2% (1)
- D2% (1)
Why each option
The Reverse Path Forwarding (RPF) check in multicast routing is designed to prevent routing loops and ensure that multicast packets are forwarded only if they arrive on the correct interface, which is the interface leading back to the source.
The RPF check validates that incoming multicast packets arrive on the network interface that the router would use to send unicast traffic back to the source, ensuring the integrity of the multicast distribution tree and preventing loops.
The RPF check drops multicast packets if they arrive on an interface that is *not* the one used to route traffic back to the source, not if they arrive on the correct one.
This option is identical to option A. Given A is correct, this too describes the same correct RPF behavior for forwarding.
This statement is incorrect as the RPF check forwards packets that arrive on the correct interface and drops those that arrive on an incorrect interface.
This describes uncontrolled flooding of multicast packets, which is precisely what the RPF check is designed to prevent by ensuring selective, loop-free forwarding.
Concept tested: Multicast Reverse Path Forwarding (RPF)
Source: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipmulti/configuration/15-mt/imc-15-mt-book/imc-cfg-rpf.html
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