Multicast Traffic Receipt Without Forwarding; Figure 2: Multicast Oif Mapping - Juniper JUNOSE 11.0.X MULTICAST ROUTING Configuration Manual

JUNOSe 11.0.x Multicast Routing Configuration Guide
forwarding. This special interface can be on a different physical port or line module
from that of the join interface.
Using this mapping function, the router can send a single copy of each multicast
stream over the special interface and the access nodes are configured to perform
any final replication to the subscribers and merge unicast and multicast data flows
onto the subscriber interfaces as necessary. See Figure 2 on page 16.

Figure 2: Multicast OIF Mapping

One disadvantage to using multicast OIF mapping is that the multicast traffic bypasses
any QoS treatment that is applied to subscriber interfaces. Configuring QoS adjustment
resolves this problem. (See Parameter Definition Attributes for QoS Administrators
Overview for additional information about configuring QoS adjustment.) With QoS
adjustment configured, when a subscriber requests to receive a multicast stream (or,
more appropriately, when an OIF is added to the mroute), the router reduces the
unicast QoS bandwidth applied to the subscriber interface (that is, the join interface)
by the amount of bandwidth for that multicast stream.

Multicast Traffic Receipt Without Forwarding

In this case, the router is not given the responsibility of forwarding multicast streams.
Instead, the service provider arranges for the router to receive the multicast streams
so the router can detect the flow and perform QoS adjustment. An OIF map is installed
that maps the traffic streams to a loopback interface configured for IGMP version
passive. This means that when the traffic is received, a null mroute is installed (that
is, an mroute with an empty OIF list) and the router applies the QoS adjustment to
the join interface. See Figure 3 on page 17.
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Configuring Multicast QoS Adjustment