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Protocol Independent Multicast (PIM) gets its
name from the fact that it is IP routing protocol independent. That
is, regardless of which unicast routing protocol(s) is (are) used to
populate the unicast routing table (including static routes), PIM
uses this information to perform multicast forwarding; hence, it is
protocol independent. Although we tend to refer to PIM as a
multicast routing protocol, it actually uses the existing unicast
routing table to perform the Reverse Path Forwarding (RPF) check
function instead of maintaining a separate multicast route table.
Because PIM doesn't have to maintain its own routing table, it
doesn't send or receive multicast route updates like other
protocols, such as Multicast Open Shortest Path First (MOSPF) or
Distance Vector Multicast Routing Protocol (DVMRP). By not having to
send multicast route updates, the PIM overhead is significantly
reduced in comparison to other multicast protocols.
PIM DM is similar to DVMRP. This protocol
works best when there are numerous members belonging to each
multimedia group. PIM floods the multimedia packet out to all
routers in the network and then prunes routers that do not support
members of that particular multicast group.
PIM dense mode is most useful when:
- Senders and receivers are in close
proximity to one another.
- There are few senders and many receivers.
- The volume of multicast traffic is high.
- The stream of multicast traffic is
constant.
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