Administrative Distance, Access Lists, Route Manipulation, and IP Multicast - CCIE Routing and Switching Exam Certification Guide - page 494

Shared trees create the root of the distribution tree somewhere between the source and the
receivers of the network. The root is called the rendezvous point (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 memory requirements of routers in the multicast network. The drawback is that the multi-
cast packets might not take the best paths to the receivers because they need to pass through the
RP. The RP router also has high memory requirements to implement its functions. PIM-Sparse
mode (PIM-SM) uses a RP. Its multicast entries are in the (*,G) format. You do not use the S
(source) because the distribution tree is not from the source.
PIM
PIM comes in sparse mode (PIM-SM) and dense mode (PIM-DM) . As the names imply, the first
operates in sparse mode and the second in dense mode.
PIM-DM uses a technique known as reverse path forwarding (RPF) . When the router receives
a packet, it forwards it out all other interfaces, which allows the data stream to reach all seg-
ments. If no hosts are members of a multicast group on any of the router's attached or down-
stream subnets, the router sends a prune message up the distribution tree to inform the upstream
router to not send packets for the multicast group. So, the analogy for PIM-DM is the push
method for sending junk mail, and you have to tell them to stop sending it.
Dense Mode Configuration
Cisco recommends configuring PIM with the sparse-dense-mode keyword. If the multicast
group is in dense mode, the interface is treated as dense. Example 10-17 shows the simple
configuration of PIM.
Example 10-17 Configuration of PIM
ip multicast-routing
!
interface ethernet 0
ip address x
ip pim sparse-dense-mode
!
interface ethernet 1
ip address y
ip pim sparse-dense-mode
If the requirement exists to strictly run in dense mode, the configuration is as shown in
Example 10-18. The only difference is that you use the dense-mode keyword instead of
sparse-dense-mode .