• 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.

Configuration:

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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