Multicast Bidirectional PIM

Hello Sean

As stated in the lesson, bidirectional PIM is more efficient for situations where you have many multicast sources and receivers, whereas PIM sparse mode is more efficient for few sources and many receivers. When we have many sources and destinations, then PIM sparse mode is ineffiecnet because:

  • the RP builds both (*,G) and (S,G) entries
  • an SPT is built towards each and every source
  • traffic only flows down the shared tree
  • PIM register/register-stop mechanisms are used to register sources to the RP

Bidirectional PIM on the other hand:

  • (S,G) entries are not built and the (*,G) entry is allowed for the shared tree
  • an SPT is never built towards the sources
  • traffic can move both up and down the shared tree
  • no PIM register/ register-stop mechanisms are used to register sources to the RP, as each source can start and stop whenever it likes (there is no way for the RP to tell a source to stop sending multicast traffic)

These characteristics make bidirectional PIM more efficient for use where you have many sources and many receivers. But you can also see that the same characteristics make sparse-mode more efficient for situations where you have only a few sources.

Yes, both would get the job done, however, you may find your network struggling to cope if you have too many sources and you are using sparse mode. Ideally, for situations where you have few sources and many receivers (broadcast IPTV for example), sparse mode should be used. For situations such as videoconferencing, you should use bidirectional mode.

I hope this has been helpful!