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ZT8101 User’s Manual 39
Switch Management and Operating Concepts
Protocol-Independent Multicast - Dense Mode (PIM-DM)
The Protocol Independent Multicast - Dense Mode (PIM-DM) protocol should be used in networks
with a low delay (low latency) and high bandwidth because PIM-DM is optimized to guarantee
delivery of multicast packets, not to reduce overhead. The switch supports PIM-DM v2.
The PIM-DM multicast routing protocol assumes that all downstream routers want to receive
multicast messages and relies on explicit prune messages from downstream routers to remove
branches from the multicast delivery tree that do not contain multicast group members.
PIM-DM has no explicit “join” messages. It relies on periodic flooding of multicast messages to all
interfaces. It then waits for the following:
• A timer to expire (the join/prune interval)
• The downstream routers to transmit explicit “prune” messages indicating that there are no
multicast members on their respective branches.
PIM-DM then removes these branches (“prunes” them) from the multicast delivery tree.
Because a member of a pruned branch of a multicast delivery tree may want to join a multicast
delivery group (at some point in the future), the protocol periodically removes the prune
information from its database and floods multicast messages to all interfaces on that branch. The
interval for removing prune information is the join/prune interval.