350-501 · Question #168
Refer to the exhibit. An engineer configured multicast routing on client's network. What is the effect of this multicast implementation?
The correct answer is B. R2 is unable to share information because the ip pirn autorp listener command is missing. For PIM Auto-RP to function correctly in a sparse-mode multicast domain, all participating routers must be able to receive RP-Discovery messages, which often requires the ip pim autorp listener command on routers not directly connected to a Mapping Agent.
Question
Refer to the exhibit. An engineer configured multicast routing on client's network. What is the effect of this multicast implementation?
Exhibit
Options
- AR2 floods information about R1 throughout the multicast domain.
- BR2 is unable to share information because the ip pirn autorp listener command is missing.
- CR1 floods information about R2 throughout the multicast domain.
- DR2 is elected as the RP for this domain.
How the community answered
(46 responses)- A4% (2)
- B70% (32)
- C9% (4)
- D17% (8)
Why each option
For PIM Auto-RP to function correctly in a sparse-mode multicast domain, all participating routers must be able to receive RP-Discovery messages, which often requires the `ip pim autorp listener` command on routers not directly connected to a Mapping Agent.
In PIM sparse mode with Auto-RP, information about RPs is distributed via specific RP-Announce and RP-Discovery messages, not by flooding general information about other routers.
The `ip pim autorp listener` command enables a PIM router to join the 224.0.1.40 group, which is where Auto-RP mapping agents send RP-Discovery messages. If this command is missing on R2, and R2 is not an RP candidate or mapping agent, it will not receive RP mapping information, thus being unable to properly participate in PIM sparse mode and share multicast information.
Similar to choice A, PIM sparse mode uses targeted mechanisms for RP information distribution, not broad flooding of router information.
The absence of the `ip pim autorp listener` command would prevent R2 from learning RP mappings, but it does not cause R2 to be elected as the RP; an explicit configuration (e.g., `ip pim rp-candidate`) is required for RP election.
Concept tested: PIM Auto-RP listener functionality
Source: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipmulti/command/imc-cr-book/imc-cr-i1.html#wp4150821568
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