User guide

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OFTWARE
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14-3
O
VERVIEW
PIM D
ENSE
M
ODE
Protocol Independent Multicast- Dense Mode (PIM-DM) is a multicast routing protocol
that is similar to DVMRP. PIM-DM routers perform reverse path multicasting (RPM).
However, instead of exchanging its own unicast route tables for the RPM algorithm,
PIM-DM uses the existing unicast route table for the reverse path. As a result, PIM-DM
requires less system memory.
PIM-DM is a broadcast and prune protocol. Using PIM-DM, multicast routes are pruned
and grafted in the same way as DVMRP.
PIM S
PARSE MODE (PIM-SM)
Unlike PIM-DM, PIM-SM is an explicit join and prune protocol, and supports shared
trees as well as shortest path trees (SPTs). The routers must explicitly join the group(s)
in which they are interested in becoming a member. This is beneficial for large networks
that have group members who are sparsely distributed.
Using PIM-SM, the router sends a join message to the rendezvous point (RP). The RP is
a central multicast router that is responsible for receiving and distributing multicast
packets.
When a router that has a multicast packet to distribute, it encapsulates the packet in a
unicast message and sends it to the RP. The RP decapsulates the multicast packet and
distributes it among all member routers.
When a router determines that the multicast rate from a particular originating router
(not the RP) has exceeded a configured threshold, that router can send an explicit join to
the originating router. Once this occurs, the receiving router gets the multicast directly
from the sending router, and bypasses the RP.
You can run either PIM-DM or PIM-SM per VLAN.
IGMP O
VERVIEW
IGMP is a protocol used by an IP host to register its IP multicast group membership
with a router. Periodically, the router queries the multicast group to see if the group is
still in use. If the group is still active, a single IP host responds to the query, and group
registration is maintained.