Pim Overview - Juniper ACX1000 Configuration Manual

ACX Series Universal Access Router Configuration Guide
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PIM Overview

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Understanding MLD
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RFC 2710, Multicast Listener Discovery (MLD) for IPv6
RFC 3810, Multicast Listener Discovery Version 2 (MLDv2) for IPv6
RFC 3810, Multicast Listener Discovery Version 2 (MLDv2) for IPv6
The predominant multicast routing protocol in use on the Internet today is Protocol
Independent Multicast, or PIM. The type of PIM used on the Internet is PIM sparse mode.
PIM sparse mode is so accepted that when the simple term "PIM" is used in an Internet
context, some form of sparse mode operation is assumed.
PIM emerged as an algorithm to overcome the limitations of dense-mode protocols such
as the Distance Vector Multicast Routing Protocol (DVMRP), which was efficient for
dense clusters of multicast receivers, but did not scale well for the larger, sparser, groups
encountered on the Internet. The Core Based Trees (CBT) Protocol was intended to
support sparse mode as well, but CBT, with its all-powerful core approach, made
placement of the core critical, and large conference-type applications (many-to-many)
resulted in bottlenecks in the core. PIM was designed to avoid the dense-mode scaling
issues of DVMRP and the potential performance issues of CBT at the same time.
Starting in Junos OS Release 15.2, only PIM version 2 is supported. In the CLI, the command
for specifying a version (1 or 2) is removed.
PIMv1 and PIMv2 can coexist on the same routing device and even on the same interface.
The main difference between PIMv1 and PIMv2 is the packet format. PIMv1 messages
use Internet Group Management Protocol (IGMP) packets, whereas PIMv2 has its own
IP protocol number (103) and packet structure. All routing devices connecting to an IP
subnet such as a LAN must use the same PIM version. Some PIM implementations can
recognize PIMv1 packets and automatically switch the routing device interface to PIMv1.
Because the difference between PIMv1 and PIMv2 involves the message format, but not
the meaning of the message or how the routing device processes the PIM message, a
routing device can easily mix PIMv1 and PIMv2 interfaces.
PIM is used for efficient routing to multicast groups that might span wide-area and
interdomain internetworks. It is called "protocol independent" because it does not depend
on a particular unicast routing protocol. Junos OS supports bidirectional mode, sparse
mode, dense mode, and sparse-dense mode.
NOTE: ACX Series routers supports only sparse mode. Dense mode on ACX
series is supported only for control multicast groups for auto-discovery of
rendezvous point (auto-RP).
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