Overview
The IPv4 implementation of PIM supports PIM dense mode, PIM sparse mode, PIM
sparse-dense mode, and PIM source-specific multicast (PIM SSM).
Figure 8 on page 78 represents how PIM builds a source, group (S,G) entry in a
source-rooted tree (SRT). When multiple routers are connected to a multiaccess network,
one router becomes the designated router. The designated router receives data from the
source on interface 1/0 and multicasts the data to its downstream neighbors on interfaces
1/1, 2/0, and 2/1. In the designated router routing table, the entry for this operation lists
the source as the IP address of the source and the group as the IP address of the multicast
group.
Figure 8: Source-Rooted Tree
Neighbors exchange hello messages periodically to determine the designated router. The
router with the highest network layer address becomes the designated router. If the
designated router subsequently receives a hello message from a neighbor with a higher
network layer address, that neighbor becomes the designated router.
PIM Dense Mode
PIM dense mode uses a reverse-path multicast, flood-and-prune mechanism. The protocol
was developed for situations that meet one or more of the following criteria:
•
Sources and receivers are close together, and there are many more receivers than
sources.
•
There is a constant stream of multicast data.
•
There is a lot of multicast data.
Dense-mode routing protocols use SRT algorithms. An SRT algorithm establishes a tree
that connects each source in a multicast group to the members of the group. All traffic
for the multicast group passes along this tree.
Copyright © 2010, Juniper Networks, Inc.78
JunosE 11.2.x Multicast Routing Configuration Guide
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