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trees that chart paths from each sender to all receivers. IP multicast routing protocols fol-
low two approaches.
The first approach assumes that the multicast group members are densely distributed
throughout the network (many of the subnets contain at least one group member), all de-
vices want to receive multicast traffic, and that bandwidth is plentiful. The approach with
dense multicast routing protocols is to flood the traffic throughout the network and then,
at the request of receiving routers, stop the flow of traffic on branches of the network that
have no members of the multicast group. Multicast routing protocols that follow this tech-
nique of flooding the network include DVMRP, Multicast Open Shortest Path First (MO-
SPF), and Protocol-Independent Multicast-Dense Mode (PIM-DM).
The second approach to multicast routing assumes that multicast group members are
sparsely distributed throughout the network, assumes received do not want to receive
multicast traffic, and that bandwidth is not necessarily widely available. Sparse mode does
not imply that the group has few members, just that they are widely dispersed. The ap-
proach with sparse multicast routing protocols is to not send traffic until it is requested by
the receiving routers or hosts. Multicast routing protocols of this type are Core-Based
Tree s (CBT ) and P rotocol-Independent Mult ic ast-Spar s e Mode (PIM-SM ). CBT is not
widely deployed and is not discussed in this topic.
Multicast Source and Shared Trees
Multicast distribution trees control the path that multicast packets take to the destination
hosts. The two types of distribution trees are source and shared. With source trees, the
tree roots from the source of the multicast group and then expands throughout the net-
work in spanning-tree fashion to the destination hosts. Source trees are also called short-
est-path trees (SPT) because they create paths without having to go through a rendezvous
point (RP). The drawback is that all routers through the path must use memory resources
to maintain a list of all multicast groups. PIM-DM uses a source-based tree.
Shared trees create the distribution tree's root somewhere between the network's source
and receivers. The root is called the RP. The tree is created from the RP in spanning-tree
fashion with no loops. The advantage of shared trees is that they reduce the memor y re-
quirements of routers in the multicast network. The drawback is that initially the multicast
packets might not take the best paths to the receivers because they need to pass through
the RP. After the data stream begins to flow from sender to RP to receiver, the routers in
the path optimize the path automatically to remove any unnecessary hops. The RP func-
tion consumes significant memory on the assigned router. PIM-SM uses an RP.
PIM
PIM comes in two flavors: sparse mode (PIM-SM) and dense mode (PIM-DM). The first
uses shared trees and RPs to reach widely dispersed group members with reasonable pro-
tocol bandwidth efficiency. The second uses source trees and reverse path forwarding
(RPF) to reach relatively close group members with reasonable processor and memory ef-
ficiency in the network devices of the distribution trees.
 
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