Designated Router; Understanding Pim Sparse Mode - Juniper ACX1000 Configuration Manual

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Designated Router

Understanding PIM Sparse Mode

Copyright © 2017, Juniper Networks, Inc.
Chapter 22: Configuring Multicast Listener Discovery and Protocol-Independent Multicast
Bidirectional PIM RPs, unlike RPs for PIM sparse mode, do not need to perform PIM
Register tunneling or other specific protocol action. Bidirectional PIM RPs implement no
specific functionality. RP addresses are simply a location in the network to rendezvous
toward. In fact, for bidirectional PIM, RP addresses need not be loopback interface
addresses or even be addresses configured on any routing device, as long as they are
covered by a subnet that is connected to a bidirectional PIM-capable routing device and
advertised to the network.
Release Description
15.2 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.
Supported IP Multicast Protocol Standards
In a PIM sparse mode (PIM-SM) domain, there are two types of designated routers to
consider:
The receiver DR sends PIM join and PIM prune messages from the receiver network
toward the RP.
The source DR sends PIM register messages from the source network to the RP.
Neighboring PIM routers multicast periodic PIM hello messages to each other every
30 seconds (the default). The PIM hello message usually includes a holdtime value for
the neighbor to use, but this is not a requirement. If the PIM hello message does not
include a holdtime value, a default timeout value (in Junos OS, 105 seconds) is used. On
receipt of a PIM hello message, a router stores the IP address and priority for that neighbor.
If the DR priorities match, the router with the highest IP address is selected as the DR.
If a DR fails, a new one is selected using the same process of comparing IP addresses.
NOTE: In PIM dense mode (PIM-DM), a DR is elected by the same process
that PIM-SM uses. However, the only time that a DR has any effect in PIM-DM
is when IGMPv1 is used on the interface. (IGMPv2 is the default.) In this case,
the DR also functions as the IGMP Query Router because IGMPv1 does not
have a Query Router election mechanism.
A Protocol Independent Multicast (PIM) sparse-mode domain uses reverse-path
forwarding (RPF) to create a path from a data source to the receiver requesting the data.
When a receiver issues an explicit join request, an RPF check is triggered. A (* ,G) PIM join
message is sent toward the RP from the receiver's designated router (DR). (By definition,
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