352-001 · Question #620
Which three different behaviors must a network designer expect when bidirectional PIM is used instead of PIM Sparse Mode? (Choose three)
The correct answer is A. The source IP addresses from the multicast senders cannot be seen in the multicast routing table D. PIMv2 BSR is not supported with bidirectional PIM E. The join messages to join a bidirectional PIM multicast group are different compared to PIM-SM. 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.
Question
Which three different behaviors must a network designer expect when bidirectional PIM is used instead of PIM Sparse Mode? (Choose three)
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
How the community answered
(31 responses)- A61% (19)
- B3% (1)
- C6% (2)
- F16% (5)
- G13% (4)
Why each option
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.
BIDIR-PIM never creates (S,G) source-specific entries in the multicast routing table - it uses only (*,G) shared tree entries for all forwarding, meaning the source IP addresses of senders are not stored or tracked in the multicast routing table.
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.
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.
PIMv2 BSR (Bootstrap Router) is not supported as an RP discovery mechanism with BIDIR-PIM; the RP must be configured statically or through a supported alternative, because BSR's RP-set distribution is incompatible with BIDIR-PIM's DF election process.
BIDIR-PIM join messages include a Bidir-capable flag (B bit) in the PIM Join/Prune message encoding to signal a bidirectional group join, making their format structurally distinct from standard PIM-SM join messages.
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.
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
Source: 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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