Configuring Multicast QoS Adjustment

When the router uses multicast OIF mapping, any multicast streams that a subscriber receives bypass any configured QoS treatment for that subscriber interface. The Multicast QoS adjust feature provides a way in which the router can account for this multicast traffic.

Note: For additional information about how to configure OIF mapping, see Configuring Group Outgoing Interface Mapping.

The following sections provide two possible configuration cases for using multicast QoS adjustment.

Note: For additional information about QoS adjustment, see Parameter Definition Attributes for QoS Administrators Overview .

Multicast OIF Mapping Case

Multicast OIF mapping enables the router to decrease the inefficiencies associated with replicating streams of multicast traffic. Using OIF maps, MLD joins that the router receives on a subscriber interface can be mapped to a special interface for 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 15.

Figure 15: 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 MLD 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 16.

Note: Ensure that PIM-SM (or any other upstream multicast protocol) is informed of the group (or source-group) interest.

Figure 16: Multicast Traffic Receipt Without Forwarding

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