PIM BiDir (PIM Bidirectional) is used for many-to-many communication. Use cases include video conferencing and gaming.

How it Works

Similar to PIM-SM, the multicast is initiated with a Shared Distribution Tree (*,G). The multicast source will send the multicast downstrean to the Rendezvous Point (RP) and the RP will deliver to the individual links that have at least one receiver. Up to this point, there is no fundamental difference between PIM-BiDir and PIM-SM.

What makes BiDir unique is that multicasting can go upstream from the receivers through a Rendezvous Point (RP).

Shortest Path Tree Switchover

As we saw with PIM-SM, the initial (*,G) setup through the RP may not be the optimal path to the source. In such a case, Shortest Path Tree Switchover will use the unicast routing table to determine the optimal path and then the path is adjusted.

Shortest Path Tree Switchover (S,G) doesn’t scale well enough for many-to-many communication so PIM-BiDir does not use it. It will continue to use the shared tree (*,G) – even if it isn’t the optimal path.

Loop Prevention

To prevent loops, BiDir doesn’t use an RPF check. It uses a designated forwarder (DF). The DF is used to create a shortest path tree to the RP.

All PIM routers on each multicast segment will hold an election to determine which PIM router will assume the designated forwarder (DF) role. The PIM router with the lowest metric will be elected the DF. If there are multiple PIM routers will the same metric – the router with the highest IP address will win.

Because only one DF can be elected per segment, there will be no loops. Embedded-RP can’t be used with BiDir.

RFC 5015: Bidirectional Protocol Independent Multicast (BIDIR-PIM)

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