Routing & Switching

IP Multicast

One-to-many delivery done properly: how group membership works with IGMP, how PIM builds distribution trees through the routed network, and how sparse mode's rendezvous points make multicast scale from market-data feeds to IPTV.

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2 chapters

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updated 31 Aug 2026
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updated 31 Aug 2026

Contents

  1. 01IP Multicast Fundamentals: Groups, IGMP and Distribution TreesIP multicast delivers one stream to many receivers by sending a single copy that routers replicate only where the tree branches, using group addresses in 224.0.0.0/4. Receivers join groups with IGMP, switches constrain flooding with IGMP snooping, and routers forward on distribution trees rooted either at the source (SPT) or a rendezvous point, always verified by an RPF check against the unicast table.
  2. 02PIM Sparse Mode and Rendezvous PointsPIM Sparse Mode builds multicast trees on an explicit-join model: last-hop routers send PIM joins toward a rendezvous point to graft onto the shared tree, sources are registered with the RP by their first-hop router, and receivers' routers then switch to the shortest-path tree to the source. The RP is pure control-plane meeting point, learned statically or via BSR/Auto-RP, with Anycast RP for redundancy.

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