Basic Pim Network Components - Juniper ACX1000 Configuration Manual

ACX Series Universal Access Router Configuration Guide

Basic PIM Network Components

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Dense mode works best in networks where few or no prunes occur. In such instances,
dense mode is actually more efficient than sparse mode.
Sparse-dense mode, as the name implies, allows the interface to operate on a per-group
basis in either sparse or dense mode. A group specified as "dense" is not mapped to
an RP. Instead, data packets destined for that group are forwarded by means of PIM
dense mode rules. A group specified as "sparse" is mapped to an RP, and data packets
are forwarded by means of PIM sparse-mode rules. Sparse-dense mode is useful in
networks implementing auto-RP for PIM sparse mode.
NOTE: On SRX Series devices, PIM does not support upstream and
downstream interfaces across different virtual routers in flow mode.
PIM dense mode requires only a multicast source and series of multicast-enabled routing
devices running PIM dense mode to allow receivers to obtain multicast content. Dense
mode makes sure that all multicast traffic gets everywhere by periodically flooding the
network with multicast traffic, and relies on prune messages to make sure that subnets
where all receivers are uninterested in that particular multicast group stop receiving
packets.
PIM sparse mode is more complicated and requires the establishment of special routing
devices called rendezvous points (RPs) in the network core. These routing devices are
where upstream join messages from interested receivers meet downstream traffic from
the source of the multicast group content. A network can have many RPs, but PIM sparse
mode allows only one RP to be active for any multicast group.
If there is only one RP in a routing domain, the RP and adjacent links might become
congested and form a single point of failure for all multicast traffic. Thus, multiple RPs
are the rule, but the issue then becomes how other multicast routing devices find the RP
that is the source of the multicast group the receiver is trying to join. This RP-to-group
mapping is controlled by a special bootstrap router (BSR) running the PIM BSR mechanism.
There can be more than one bootstrap router as well, also for single-point-of-failure
reasons.
The bootstrap router does not have to be an RP itself, although this is a common
implementation. The bootstrap router's main function is to manage the collection of RPs
and allow interested receivers to find the source of their group's multicast traffic. PIM
bootstrap messages are sourced from the loopback address, which is always up. The
loopback address must be routable. If it is not routable, then the bootstrap router is
unable to send bootstrap messages to update the RP domain members. The
bootstrap command displays only those bootstrap routers that have routable loopback
addresses.
PIM SSM can be seen as a subset of a special case of PIM sparse mode and requires no
specialized equipment other than that used for PIM sparse mode (and IGMP version 3).
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