Juniper ACX1000 Configuration Manual page 731

Copyright © 2017, Juniper Networks, Inc.
Chapter 22: Configuring Multicast Listener Discovery and Protocol-Independent Multicast
PIM operates in several modes: bidirectional mode, sparse mode, dense mode, and
sparse-dense mode. In sparse-dense mode, some multicast groups are configured as
dense mode (flood-and-prune, [S,G] state) and others are configured as sparse mode
(explicit join to rendezvous point [RP], [*,G] state).
PIM drafts also establish a mode known as PIM source-specific mode, or PIM SSM. In
PIM SSM there is only one specific source for the content of a multicast group within a
given domain.
Because the PIM mode you choose determines the PIM configuration properties, you first
must decide whether PIM operates in bidirectional, sparse, dense, or sparse-dense mode
in your network. Each mode has distinct operating advantages in different network
environments.
In sparse mode, routing devices must join and leave multicast groups explicitly.
Upstream routing devices do not forward multicast traffic to a downstream routing
device unless the downstream routing device has sent an explicit request (by means
of a join message) to the rendezvous point (RP) routing device to receive this traffic.
The RP serves as the root of the shared multicast delivery tree and is responsible for
forwarding multicast data from different sources to the receivers.
Sparse mode is well suited to the Internet, where frequent interdomain join messages
and prune messages are common.
NOTE: On all the EX series switches (except EX4300 and EX9200),
QFX5100 switches, and OCX series switches, the rate limit is set to 1pps
per SG to avoid overwhelming the rendezvous point (RP), First hop router
(FHR) with PIM-sparse mode (PIM-SM) register messages and cause CPU
hogs. This rate limit helps in improving scaling and convergence times by
avoiding duplicate packets being trapped, and tunneled to RP in software.
(Platform support depends on the Junos OS release in your installation.)
Bidirectional PIM is similar to sparse mode, and is especially suited to applications that
must scale to support a large number of dispersed sources and receivers. In bidirectional
PIM, routing devices build shared bidirectional trees and do not switch to a source-based
tree. Bidirectional PIM scales well because it needs no source-specific (S,G) state.
Instead, it builds only group-specific (* ,G) state.
Unlike sparse mode and bidirectional mode, in which data is forwarded only to routing
devices sending an explicit PIM join request, dense mode implements a flood-and-prune
mechanism, similar to the Distance Vector Multicast Routing Protocol (DVMRP). In
dense mode, a routing device receives the multicast data on the incoming interface,
then forwards the traffic to the outgoing interface list. Flooding occurs periodically and
is used to refresh state information, such as the source IP address and multicast group
pair. If the routing device has no interested receivers for the data, and the outgoing
interface list becomes empty, the routing device sends a PIM prune message upstream.
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