received on the correct interface, the router discards the packet. Only packets received
on the correct RPF interface are considered for forwarding to downstream receivers.
When operating in sparse-mode, the routers perform an RPF lookup to identify the
upstream router from which to request the data and then send join messages for the
multicast stream only to that router.
When operating in dense-mode, routers that have multiple paths to the source of the
multicast stream initially receive the same stream on more than one interface. In this
case, the routers perform an RPF lookup to identify multicast data streams that are not
arriving on the best path and send prune messages to terminate these flows.
The RPF lookup need not always be towards the source of the multicast stream. The
lookup is done towards the source only when the router is using a source-rooted tree to
receive the multicast stream. If the router uses a shared tree instead, the RPF lookup is
toward a rendezvous point and not toward the source of the multicast stream.
Multicast Packet Forwarding
Multicast packet forwarding is based on the source (S) of the multicast packet and the
destination multicast group address (G). For each (S,G) pair, the router accepts multicast
packets on an incoming interface (IIF), which satisfies the RPF check (RPF-IIF). The router
drops packets received on IIFs other than the RPF-IIF and notifies the routing protocols
that a packet was received on the wrong interface.
The router forwards packets received on the RPF-IIF to a list of outgoing interfaces (OIFs).
The list of OIFs is determined by the exchange of routing information and local group
membership information. The router maintains mappings of (S,G, IIF) to {OIF1, OIF2…}
in the multicast routing table.
You can enable two or more multicast protocols on an IIF. However, only one protocol
can forward packets on that IIF. The protocol that forwards packets on an IIF owns that
IIF. A multicast protocol that owns an IIF also owns the (S,G) entry in the multicast routing
table.
Platform Considerations
For information about modules that support IP multicasting on the ERX7xx models,
ERX14xx models, and the Juniper Networks ERX310 Broadband Services Router:
•
See ERX Module Guide, Table 1, Module Combinations for detailed module specifications.
•
See ERX Module Guide, Appendix A, Module Protocol Support for information about the
modules that support IP multicasting.
For information about modules that support IP multicasting on the Juniper Networks
E120 and E320 Broadband Services Routers:
•
See E120 and E320 Module Guide, Table 1, Modules and IOAs for detailed module
specifications.
•
See E120 and E320 Module Guide, Appendix A, IOA Protocol Support for information
about the modules that support IP multicasting.
5Copyright © 2010, Juniper Networks, Inc.
Chapter 1: Configuring IPv4 Multicast
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