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JNCIE-SP: Multicast

JNCIE-SP · Work through your exam, one topic at a time.

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Multicast

What this domain tests

Multicast questions test IGMP, PIM-SM and SSM, RP placement, MSDP, and multicast VPNs. The exam focuses on RPF, shared versus source trees, and how multicast is tunneled over MPLS.

Core rules

  • IGMP is configured on receiver-facing interfaces: set protocols igmp interface ge-0/0/0.0 version 3. The querier is elected by lowest IP address.
  • PIM-SM builds a shared tree rooted at the RP; the last-hop router joins (*,G), then may switch to the shortest-path tree (S,G). spt-threshold infinity keeps traffic on the shared tree.
  • RPF is the core check: multicast traffic must arrive on the interface the unicast route uses to reach the source. show pim rpf displays the expected interface.
  • PIM-SSM requires IGMPv3 and a source-specific join; no RP is needed. The default SSM range is 232.0.0.0/8.
  • Anycast RP uses the same RP address on multiple routers with MSDP peering to exchange source-active messages.
  • MVPN uses a provider tunnel (PIM/GRE, mLDP, or RSVP-TE P2MP) to carry customer multicast. NG-MVPN uses BGP MVPN routes for autodiscovery.

Worked example

An RP is configured statically, and the RP must be reachable via the IGP:

set protocols pim rp static address 10.0.0.1
set protocols pim interface all mode sparse

Reasoning: every router must map the group to the same RP. If 10.0.0.1 is not reachable in inet.0, PIM joins fail because there is no route to the RP. Verify with show pim rp and show pim join.

Traps

  • RPF failure is usually a unicast routing problem, not a PIM problem.
  • MSDP is for anycast RP and source discovery; it does not replace PIM.
  • MBGP (family inet multicast) can change RPF lookup; check the multicast RIB if unicast and multicast topologies diverge.
  • A PIM-SM router with no RP for a group cannot build a shared tree.