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which the packet was received. Subsequent prune
messages are used to prune branches of the delivery tree
that are either not on the shortest path back to the multicast
source, or that have no active multicast group members. A
'graft' message is added that allows a previously pruned
branch of the multicast delivery tree to be reactivated. This
allows for lower latency when a leaf router adds a new
member to a multicast membership group. Graft messages
are forwarded one hop (one router) back at a time toward a
multicast source until they reach a router that is on an active
branch of the multicast delivery tree.
If there is more than one multicast router on a network, the
one that has the shortest path back to the multicast source is
elected to forward multicast packets from that source. All
other routers will discard multicast packets from that source.
If two multicast routers on a network have the same distance
back to a multicast source, the router with the lowest IP
address is elected.
DVMRP also supports tunnel interfaces, where two multicast
routers are connected through a router that cannot process
multicast packets. This allows multicast packets to cross
networks with routers that are not multicast-aware.
172 VH-2402-L3 Management Guide
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