• RFC 4601
  • Uses ‘Pull’ model or explicit join.
  • Uses two tree types:
    • RPT – Shared tree
    • SPT – Shortest Path tree
    • NOTE
      • Dense mode uses only SPT
  • More scalable and usually better design choice then Dense mode.
    • Dense mode basically legacy.
  • Multicast tree determines how traffic is routed from sender to receiverS.
  • Source based tree
    • Uses shortest path from sender to receiver.
    • Dense mode or sparse mode
  • Shared trees
    • Uses shortest path from sender Rendezvous Point, then shortest path from RP to receiver.
    • Sparse mode only
    • Used for the following:
      • Eliminate flooding
      • Eliminate pruning
      • Making routing table more scalable.
  • Sparse Mode Operations:
    • Discover PIM neighbors and elect Designated Router.
    • Discover RP
    • Tell RP about sources.
    • Tell RP about receivers.
    • Build Shared Tree from Sender to Receivers through RP.
    • Join shortest path tree.
    • Leave shared tree.
    • Multicast Table Maintenance.
  • Rendezvous Point:
    • Used as reference point for root of shared tree.
    • Learns about sources from Unicast PIM Register messages.
      • Register message gives PIM an (S,G)
    • Learns about receivers through PIM Join messages.
      • Join message tells RP to add interface to OIL for (*,G)
    • Used to merge the two trees together.
    • Without RP, there will be zero registers and joins in PIM sparse mode.
    • As Root of all shared trees, RP must know all sources.
    • More operations:
      • When first hop router connected to sender hears traffic, a unicast Register is sent to RP.
        • Only DR sends register if multiple multicast routers available.
      • If RP accepts, it acknowledges with Register Stop and inserts (S,G) into Multicast table.
        • At this point only DR and RP know (S,G) mapping.
      • PIM Join
        • When last hop router receives IGMP Report, a PIM Join is generated up reverse path (tree) to RP.
        • All routers in reverse path install (*,G) and forward the Join hop-by-hop to the RP.
        • At this point all downstream routers toward receiver know (*,G).
      • Merging Trees
        • Once RP knows (S,G) from sender and (*,G) from receivers, RP sends PIM Join up reverse path to source.
        • All routers in between RP and source install (*,G) with outbound interface list pointing towards RP.
        • Once (S,G) begins, tree is built end to end through RP.
      • SPT
        • Shared tree is made of two Shortest Path Trees.
          • SPT from RP to sender
          • SPT from receiver to RP
          • SPT from receiver to sender (both combined) may not be same as shared tree.
            • result is shared tree not optimal.
            • Fix
              • Last hop router joins SPT to source with (S,G) Join.
              • Leaves RPT by sending (*,G) Prune to RP
            • Can be modified with ‘ip pim spt-threshold’.
  • Routing Table Maintenance:
    • PIM Dense and PIM Sparse use State Refresh to ensure feeds to not timeout.
      • (*,G) join sent to RP or up SPT to refresh the OIL.
    • Sparse Prune message can be used to speed up state information timeout if IGMP Leave is heard from end point.

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