Multicast VoD Design Across Multiple ASes for High Availability

300-420 By islever Editorial Last updated

Design multicast services (SSM, PIM bidirectional, MSDP, service-reflection) Describe multicast routing concepts (source trees, shared trees, RPF, rendezvous points)

Short answer

D MSDP enables RPs in different ASes to exchange active source information, allowing receivers in one AS to receive VoD streams from sources in another AS when local sources fail.

Question

An engineer is designing a multicast network for a company specializing in VoD content. Receivers are across the Internet, and for performance reasons, the
multicast framework must be close to the receivers within each AS. For high availability, if the sources in one AS are no longer available, the receivers of that AS
must be able to receive the VoD content from the sources in another AS. Which feature must the design include?
  1. SSM
  2. anycast RP
  3. bidirectional PIM
  4. MSDP Correct Answer

Key Takeaways

The exam tests whether you recognize that inter-AS multicast source sharing requires MSDP, and the common trap is confusing anycast RP (an intra-domain RP redundancy method) with the inter-domain source discovery mechanism that MSDP provides.

This question tests how to enable inter-AS multicast source discovery when each Autonomous System runs a local RP for performance. MSDP is the feature that allows RPs in different ASes to exchange active source information, ensuring receivers in one AS can receive VoD streams from sources in another AS if local sources fail.

Common mistake: Anycast RP is a common wrong answer because it also involves RP redundancy and is sometimes paired with MSDP. However, anycast RP alone is an intra-domain RP redundancy technique and does not natively enable cross-AS source discovery — the scenario requires RPs in separate ASes to share source information, which is exactly what MSDP does.

Expert Analysis

Why the Answer Is Correct

MSDP (Multicast Source Discovery Protocol) is the correct answer because the scenario requires each AS to have its own local RP for performance while enabling cross-AS source discovery for high availability. MSDP peers RPs in different ASes and advertises active multicast sources (via Source-Active messages) from one RP domain to another. When sources in AS1 fail, the local RP in AS1 learns about available sources in AS2 through its MSDP peer, allowing receivers in AS1 to receive VoD content from AS2 sources. This is the classic MSDP use case: connecting multiple PIM-Sparse Mode domains across Autonomous Systems for source sharing and redundancy.

Why the Other Options Are Wrong

Anycast RP (Option B) provides RP redundancy within a single PIM-SM domain by assigning the same IP address to multiple RPs. While anycast RP can be deployed with MSDP internally to synchronize sources between RPs, anycast RP alone does not inherently enable inter-AS source discovery across separate ASes with independent multicast domains. SSM (Option A) eliminates the need for an RP entirely by using source-specific joins, but it does not provide the automatic inter-AS source sharing mechanism described. Bidirectional PIM (Option C) optimizes multicast forwarding when receivers greatly outnumber sources but provides no inter-domain source discovery capability.

Community Comment Notes

Several community members argued for option B (anycast RP). As iSDA69 noted, anycast RP allows "clients of one AS to contact the same (other AS) RP IP address when the local RP fails." However, this conflates RP redundancy with inter-AS source sharing — the question describes independent local RPs per AS, not a shared anycast RP address. As NoHombre correctly explained, "MSDP connects multiple PIM-SM domains by allowing RPs to" exchange source information, which is precisely what the high-availability requirement in this scenario demands. PicoOstrava acknowledged MSDP is used to propagate sources across domains but incorrectly claimed anycast RP better handles HA — the HA mechanism here is source sharing across ASes, not RP failover.

Exam Strategy

When a question mentions multiple Autonomous Systems and inter-domain multicast with RPs close to receivers, immediately think MSDP — it is the only listed option specifically designed for inter-AS source sharing. Anycast RP is intra-domain RP redundancy, so look for AS-boundary language to distinguish the two.

Frequently Asked Questions

Why isn't anycast RP correct when the question mentions high availability across ASes?

Anycast RP provides RP redundancy within a single PIM-SM domain by sharing one IP address among multiple RPs. The question describes independent ASes each with a local RP, requiring inter-AS source discovery, which is MSDP's function.

How does MSDP enable receivers in one AS to receive streams from sources in another AS?

MSDP peers RPs across ASes and sends Source-Active messages advertising active multicast sources. When a local RP learns of a remote source, it can relay that information to local receivers so they can join the remote stream.

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Discussion 14 contributions

Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.

From public community threads 14

  1. PicoOstrava 👍 4 Answered B
    option D, is also used to propagate multicast sources across different domains. However, it is primarily used for source discovery, and it does not directly handle high availability as effectively as anycast RP in this context.
  2. neiker45 👍 4 Answered D
    MSDP is required for inter-domain multicasting
  3. NoHombre 👍 3 Answered D
    You have receivers spread across multiple ASes (interdomain) and you want the multicast control plane close to the receivers in each AS (i.e., each AS runs its own PIM-SM with a local RP). For high availability, if sources in AS1 fail, receivers in AS1 must be able to learn and join sources in AS2 automatically. That is exactly what MSDP does: it peers the RPs of different domains and advertises Source-Active (SA) messages. Using SA information, an RP in one AS learns about active sources in the other AS and can trigger joins toward those sources. This enables interdomain source discovery and failover while keeping the PIM framework (RP) local to each AS for performance.
  4. anaq87 👍 3 Answered D
    The correct answer is D. MSDP (Multicast Source Discovery Protocol). Why MSDP is required: The design involves multiple PIM-Sparse Mode domains (one in each AS), where the RPs are local to each receiver AS for performance. When sources in one AS fail, receivers in that AS must still be able to get VoD streams from sources in a different AS. MSDP connects multiple PIM-SM domains by allowing RPs to share active source information (the “(S, G)” state) across AS boundaries. With MSDP, if a source goes down in one AS, the RP in that AS can learn about active sources in another AS via MSDP, allowing receiver RPs to join alternate sources dynamically.
  5. iSDA69 👍 2 Answered B
    I Think the right answer should be B Anycast RP. That's because this is the feature that has to be enabled to allow the clients of one AS to contact the same (other AS) RP IP address when the local RP fails. This will allow the clients to join the multicast sources of the other AS. MSDP in this question is considered enabled because we are in a multiple AS scenario and has the function (as usual) to advertise the sources between AS.
Show 9 more comments
  1. Reinier_veen 👍 2
    Info about AnycastRP: https://www.cisco.com/c/en/us/td/docs/ios/solutions_docs/ip_multicast/White_papers/anycast.html
  2. namibdigger 👍 2 Answered B
    Difficult - it reminds me of anycast in general with BGP 'close to nearest' - thats why i opt for B
  3. cc37f90 👍 2 Answered D
    The MSDP Solution: MSDP runs between RPs in different Autonomous Systems!
  4. sola123 👍 2 Answered B
    it says multicast feature not multicast mode
  5. vangio 👍 2
    Correct B
  6. G_Singh12 👍 1 Answered D
    The key requirement is that receivers located inside one Autonomous System (AS) must be able to switch to multicast sources located in a completely different AS if their local sources become unavailable. MSDP • is explicitly designed to handle this by peering Rendezvous Points (RPs) across different autonomous systems or domains. When sources become active or change locations across the Internet/AS boundaries, MSDP shares Source Active (SA) messages between the RPs. This mechanism dynamically informs the remote AS where the alternative active sources are located so the tree can be re-routed.
  7. drinu89 👍 1 Answered B
    Multiple RPs share the same RP address. Sources and receivers use the nearest RP according to routing metrics. Provides RP load balancing and redundancy This question includes locality + HA + cross-AS source availability, which is the classic Anycast RP + MSDP design. Because the answer choices allow only one choice, Cisco is typically asking for the feature, not the helper protocol
  8. TheGorn 👍 1
    This questions sucks. Looks like both B and D could be considered pieces of the same solution. https://www.cisco.com/c/en/us/td/docs/ios/solutions_docs/ip_multicast/White_papers/anycast.html
  9. TheGorn 👍 1
    Going with D though as one is dependent on the other and the wording states "must include".

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