350-501 Question #139: Real Exam Question with Answer & Explanation

Question

What causes multicast traffic to permanently stay on the shared tree and not switch to the source tree?

Options

• AThe SPT threshold is set to infinity.
• BThe RP IP address is configured incorrectly.
• CThe RP announcements are being filtered.
• DSSM range is being used.

Explanation

Multicast traffic can permanently remain on the shared tree if the Rendezvous Point (RP) announcements, which are critical for source discovery, are being filtered, preventing the successful switch to a more optimal source tree.

Common mistakes.

• A. Setting the SPT threshold to infinity would also cause traffic to stay on the shared tree, but filtering RP announcements is a common misconfiguration that leads to the same outcome due to lack of source information for SPT initiation.
• B. An incorrectly configured RP IP address would likely prevent the shared tree from forming or functioning properly in the first place, rather than just preventing an SPT switch.
• D. Source Specific Multicast (SSM) explicitly uses source trees and does not involve RPs or shared trees, so traffic would not be on a shared tree at all in this scenario.

Concept tested. PIM-SM Shared Tree vs. Source Tree

Reference. https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipmulti/command/imc-cr-book/imc-cr-p1.html#wp2213898236

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