352-001 Question #620: Real Exam Question with Answer & Explanation

Question

Options

• AThe source IP addresses from the multicast senders cannot be seen in the multicast routing table
• BThe RPF check does not prevent routing loops when bidirectional PIM is used
• CMany possible rendezvous point can be used for bidirectional PIM as compared to PIM Sparse
• DPIMv2 BSR is not supported with bidirectional PIM
• EThe join messages to join a bidirectional PIM multicast group are different compared to PIM-SM
• FNo rendezvous point is required when bidirectional PIM is used
• GAuto-RP is not supported with bidirectional PIM

Explanation

Bidirectional PIM (BIDIR-PIM) differs from PIM Sparse Mode in that it uses only shared tree state, has specific RP discovery constraints, and uses modified join message formats. Designers must account for these differences when planning multicast deployments.

Common mistakes.

• B. BIDIR-PIM uses a Designated Forwarder (DF) election on each link to control forwarding toward the RP, which does prevent routing loops - the loop prevention mechanism is different from RPF but is still present and effective.
• C. BIDIR-PIM does not inherently support more RPs than PIM-SM; both modes support one RP per group address, and the number of configurable RPs is independent of the PIM forwarding mode used.
• F. BIDIR-PIM still requires a configured Rendezvous Point to anchor the shared tree and drive the DF election process; the distinction from PIM-SM is that traffic flows bidirectionally on the shared tree, not that the RP is absent.
• G. Auto-RP is supported with BIDIR-PIM in applicable Cisco IOS implementations, whereas BSR (choice D) is the RP discovery mechanism that is specifically unsupported with BIDIR-PIM.

Concept tested. Bidirectional PIM behavioral differences vs PIM Sparse Mode

Reference. https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipmulti_pim/configuration/xe-16/imc-xe-16-book/imc-bidir-pim.html

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