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same interface used for routing unicast packets to the multicast source
network. If it is not, the router drops the packet and sends a prune
message back out the source interface. If it is the same interface used by
the unicast protocol, then the router forwards a copy of the packet to all
the other interfaces for which is has not already received a prune message
for this specific source-group pair.
DVMRP holds the prune state for about two hours, while PIM-DM holds
it for only about three minutes. This results in more flooding than
encountered with DVMRP, but this the only major trade-off for the lower
processing overhead and simplicity of configuration for PIM-DM.
Configuring Global PIM-DM Settings
PIM-DM is used to route multicast traffic to nodes which have requested a
specific multicast service via IGMP. It uses the router’s unicast routing
table to determine if the interface through which a packet is received
provides the shortest path back to the source. This is done on a per hop
basis back toward the source of the multicast delivery tree. PIM-DM uses
three different techniques to dynamically reconfigure the multicast
spanning tree: broadcasting, pruning, and grafting.
To use PIM-DM, you must enable it globally for the router as described
below, and for each interface that will support multicast routing as
described in the next section. Also note that IGMP must be enabled to
allow the router to determine the location of group members.
Web – Click Routing Protocol, PIM-DM, General Settings. Enable or
disable PIM-DM globally for the router, and click Apply.