Interdomain Multicast Protocols: MP-BGP and MSDP

300-420 By islever Editorial Last updated

Design multicast services (SSM, PIM bidirectional, MSDP, service-reflection)

Short answer

CD MP-BGP and MSDP are the two protocols deployed to achieve communication between multicast sources and receivers in an interdomain environment.

Question

When designing interdomain multicast, which two protocols are deployed to achieve communication between multicast sources and receivers? (Choose two.)
  1. IGMPv2
  2. BIDIR-PIM
  3. MP-BGP Correct Answer
  4. MSDP Correct Answer
  5. MLD

Key Takeaways

The key trap is confusing intra-domain protocols like BIDIR-PIM or IGMP with inter-domain mechanisms; only MP-BGP and MSDP bridge the gap between separate multicast domains.

Learn how MP-BGP and MSDP enable interdomain multicast communication by exchanging RP and source information between autonomous systems.

Common mistake: Choosing BIDIR-PIM (B) is a common mistake because it is a valid multicast protocol, but it operates primarily within a domain. It lacks the native capability to synchronize RPs or sources across AS boundaries without the aid of MP-BGP or MSDP.

Expert Analysis

The question specifically asks about interdomain multicast, which refers to communication between different Autonomous Systems (AS). The boundary between intra-AS and inter-AS multicast is the critical distinction here.

MP-BGP (Multiprotocol BGP): Standard BGP does not carry multicast routing information. MP-BGP extends BGP to exchange multicast routes (specifically RP addresses) between PE routers in different ASes. This allows receivers in one domain to discover sources in another by exchanging Rendezvous Point (RP) information via the underlay BGP session. This is the standard mechanism for interdomain PIM-SM.

MSDP (Multicast Source Discovery Protocol): MSDP peers are configured between RPs in different domains. When a source registers with its local RP, that RP uses MSDP to inform remote RPs about the active source. This enables receivers in other domains to join the source's tree.

Why BIDIR-PIM is incorrect: While Bidirectional PIM supports efficient many-to-many communication within a domain, it typically relies on a single shared RP per domain. Without MP-BGP or MSDP to coordinate RPs across domains, BIDIR-PIM cannot function effectively for interdomain traffic because there is no mechanism to establish the necessary cross-domain connectivity or RP synchronization. Comments suggesting BIDIR-PIM often confuse intra-domain design with inter-domain requirements.

Official Reference

https://www.cisco.com/c/en/us/td/docs/ios/solutions_docs/ip_multicast/Phase_1/mcstmsdp/mcst_p1.html

This link opens the official vendor documentation in a new tab.

Exam Strategy

When you see 'interdomain' or 'between autonomous systems' in a multicast question, immediately eliminate Layer 2 or intra-AS protocols. Focus on mechanisms that bridge domains: MP-BGP for control plane RP distribution and MSDP for source discovery. If both options appear, they are likely the pair.

Frequently Asked Questions

What is the difference between standard BGP and MP-BGP in multicast?

Standard BGP carries unicast routing prefixes. MP-BGP (Multiprotocol BGP) is an extension that can carry multiple address families, including IPv4 multicast routes and RP addresses, which is essential for interdomain multicast.

Can MSDP work without MP-BGP?

Yes, MSDP can be used independently between RPs in different domains if the RPs are manually configured or learned through other means. However, in large-scale MPLS networks, MP-BGP is often preferred for distributing RP addresses automatically.

More 300-420 FAQ →

Community Votes

D
50%
C
43%
B
7%

50% of anonymous learners picked answer D. Votes are pick records left by other test-takers — they are not the verified answer.

Discussion 11 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 11

  1. Lungful 👍 3
    I vote C+D based on: https://www.cisco.com/c/en/us/td/docs/ios/solutions_docs/ip_multicast/Phase_1/mcstmsdp/mcst_p1.html
  2. XalaGyan 👍 2
    Answers B and D When designing interdomain multicast, two protocols that are typically deployed to achieve communication between multicast sources and receivers are: BIDIR-PIM (Option B): BIDIR-PIM (Bi-Directional PIM) is a multicast routing protocol that allows multicast traffic to flow in both directions between domains. It uses a shared tree architecture to support interdomain multicast communication. MSDP (Option D): MSDP (Multicast Source Discovery Protocol) is used to exchange information between domains about multicast sources. MSDP allows routers in one domain to learn about multicast sources in other domains and to forward traffic to those sources. MSDP operates at the edge of each domain and is used in conjunction with BIDIR-PIM. In summary, BIDIR-PIM and MSDP are two protocols that are deployed to achieve interdomain multicast communication between multicast sources and receivers.
  3. namibdigger 👍 2
    MSDP (Multicast interdomain discovery protocol) should exchange routing information using BGP (Student learning guide v1.4. p 274)
  4. cerifyme85 👍 1
    https://www.arista.com/en/um-eos/eos-multicast-source-discovery-protocol-msdp#:~:text=configuring%20MSDP%20speakers.-,Network%20Configuration,messages%20are%20MSDP%20control%20messages%20that%20peers%20exchange%20during%20peering%20sessions,-.
  5. _MaGIC_13 👍 1
    Multicast Protocols spoken at Source Segments Intradomain - PIM & Variants Interdomain - MP-BGP & MSDP Receiver segments use IGMP or MLD (Multicast Listener Discovery IPv6)
Show 6 more comments
  1. _MaGIC_13 👍 1
    Multicast Protocols | Interdomain Protocols MP-BGP & MSDP Keyword is Inter meaning between domains not within.
  2. Clauster 👍 1
    You are incorrect, i just asked bard and it gave me B and C as the correct answers
  3. GustavoF 👍 1
    BGP itself differs from MP-BGP operation. In my option B and D are correct.
  4. Yeti56 👍 1
    Interdomain multicast networks such as PIM-SM, MBGP, and MSDP
  5. G_Singh12 👍 1
    C & D are correct
  6. Clauster 👍 1
    Sorry C and D

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