300-420 · Question #341
What are two purposes of the RPF check in multicast routing? (Choose two.)
The correct answer is A. to ensure that multicast packets are forwarded if they arrived on the interface used to route traffic C. to ensure that multicast packets are dropped if they arrived on the interface used to route traffic to. The Reverse Path Forwarding (RPF) check in multicast routing is a loop prevention mechanism that validates the source path of multicast packets. It ensures that multicast packets are forwarded only if they arrive on the interface used to route traffic back to the source and are…
Question
What are two purposes of the RPF check in multicast routing? (Choose two.)
Options
- Ato ensure that multicast packets are forwarded if they arrived on the interface used to route traffic
- Bto ensure that multicast packets are forwarded if they arrived on the interface used to route traffic
- Cto ensure that multicast packets are dropped if they arrived on the interface used to route traffic to
- 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
(60 responses)- A95% (57)
- B2% (1)
- D2% (1)
- E2% (1)
Why each option
The Reverse Path Forwarding (RPF) check in multicast routing is a loop prevention mechanism that validates the source path of multicast packets. It ensures that multicast packets are forwarded only if they arrive on the interface used to route traffic back to the source and are dropped otherwise.
The RPF check ensures that a multicast packet is forwarded when it arrives on the interface that the unicast routing table indicates as the shortest path back to the source of the multicast traffic. This successful RPF check validates the packet's origin and prevents loops.
This statement is identical to choice A, which is already selected as correct, implying a redundancy in the question's options.
If a multicast packet arrives on an interface that is not the shortest path back to the source as determined by the unicast routing table, the RPF check fails, and the packet is dropped. This mechanism is crucial for preventing multicast forwarding loops and ensuring efficient multicast distribution.
This statement is identical to choice C, which is already selected as correct, implying a redundancy in the question's options.
Forwarding multicast packets out all interfaces regardless of arrival interface or RPF check would lead to uncontrolled flooding and loops, which RPF is specifically designed to prevent.
Concept tested: Multicast RPF check purpose
Source: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipmulti/command/imc-cr-book/imc-cr-l2.html
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