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Cisco SPRI 300-510: Multicast Routing Implementation (PIM-SM, PIM-SSM, MSDP, mLDP)

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Multicast Routing Implementation (PIM-SM, PIM-SSM, MSDP, mLDP)

What this domain tests

Multicast Routing Implementation tests PIM sparse mode shared and source trees, source-specific multicast, MSDP synchronization between RPs, and mLDP for label-switched multicast transport. Expect RP design, RPF reasoning, and anycast RP troubleshooting.

PIM-SM

  • The RP is the root of the shared tree. Receivers send (*,G) joins toward the RP; sources register unicast to the RP, and the RP sends (S,G) joins toward the source.
  • The last-hop router switches to the shortest-path tree after receiving data; the RP sends a register-stop once the source is reachable.
  • The DR on a LAN is elected by highest priority, then highest IP; PIM asserts resolve duplicate forwarders.
  • RPF checks use the unicast route to the source or RP, not the multicast route.

PIM-SSM

  • SSM builds an (S,G) tree directly from the receiver; there is no RP and no register process.
  • IGMPv3 supplies the source address; the default SSM range is 232.0.0.0/8.
  • IGMPv2 receivers can use SSM mapping to translate a group to a source.

MSDP

  • MSDP peers use TCP 639 and exchange source-active (SA) messages between RPs.
  • The receiving RP runs an RPF check on the SA toward the originating RP; a failed check drops the SA.
  • Anycast RP uses the same RP address on multiple routers plus MSDP to synchronize active sources.
  • originator-id ensures the SA carries the anycast RP address rather than the physical peer address.

mLDP

  • mLDP uses LDP messages to build P2MP and MP2MP LSPs, with a FEC carrying the root node and opaque value.
  • The root initiates the LSP; labels are distributed downstream. mLDP is common for mVPN traffic.

Worked example

RP1 and RP2 both configure loopback 10.0.0.100/32 as the RP and advertise it in the IGP. RP1 peers with RP2 using:

ip msdp peer 10.1.1.2 connect-source Loopback1
ip msdp originator-id Loopback0

A source registers with RP1; RP1 sends an SA to RP2. A receiver joins RP2, which now knows the active source and sends an (S,G) join toward it. The originator-id keeps the SA's RP field at 10.0.0.100 so the RPF check succeeds even though the TCP peer address is 10.1.1.2.

Traps

  • MSDP SA fails RPF when the peer address or route is wrong.
  • PIM-SM without a consistent RP set builds no shared tree.
  • SSM groups outside the configured range fall back to PIM-SM.
  • mLDP does not replace multicast routing; it provides the transport.