• RFC 5015
  • Traditional Sparse Mode forms two trees.
    • Unidirectional SPT from source to RP
    • Undirectional shared tree from RP to receivers.
  • Results in (*,G) and (S,G) control plane.
    • Doesn’t scale well.
  • Bidirectional PIM solves this by only allowing the Shared Tree (*,G) and never a SPT (S,G).
  • Operations:
    • Define an RP and group range as bidirectional.
      • Stops formation of (S,G) for range.
    • Build single (*,G) tree towards RP
      • Traffic flows upstream from source to RP
      • Traffic flows downstream from R to receivers
    • Removes PIM Register process
      • Traffic from sources always flow to RP.
    • Uses Designated Forwarder for loop prevention.
  • Bidir Designated Forwarder:
    • One DF is elected per PIM segment
      • Lowest metric to RP wins.
      • Highest IP in tie.
    • Only DF can forward traffic upstream towards RP.
    • All other interfaces in OIL are downstream facing.
    • Removes the need for RPF check.
      • Due to this all routers must agree on Bidir or loops can occur.


In this topology we’re going to setup R1 as the Rendezvous Point.

Bidirectional PIM first needs to be enabled globally, and then added on to the rp-address command. This needs to get turned on for every router in the path.

Now on R8 we’re going to join a group.

After the group Join reached the RP, R1 now sees the receiver/(*,G).

We’re going to now setup a continuous ping on R9 to the group address and see what the mroute shows on R1.

Which is still only showing the (*,G) because this is Bidirectional PIM. The full SPT does not exist, will not form.

Entire goal is to reduce the number of senders in the Multicast Table.

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