Multicast Forwarding - Cisco Nexus 7000 Series Configuration Manual

Overview

Multicast Forwarding

Figure 4: Bidirectional Shared Tree
Multicast Forwarding
Because multicast traffic is destined for an arbitrary group of hosts, the router uses reverse path forwarding
(RPF) to route data to active receivers for the group. When receivers join a group, a path is formed either
toward the source (SSM mode) or the RP (ASM or Bidir mode). The path from a source to a receiver flows
in the reverse direction from the path that was created when the receiver joined the group.
For each incoming multicast packet, the router performs an RPF check. If the packet arrives on the interface
leading to the source, the packet is forwarded out each interface in the outgoing interface (OIF) list for the
group. Otherwise, the router drops the packet.
Note In Bidir mode, if a packet arrives on a non-RPF interface, and the interface was elected as the designated
forwarder (DF), then the packet is also forwarded in the upstream direction toward the RP.
The figure below shows an example of RPF checks on packets coming in from different interfaces. The packet
that arrives on E0 fails the RPF check because the unicast route table lists the source of the network on interface
E1. The packet that arrives on E1 passes the RPF check because the unicast route table lists the source of that
network on interface E1.
Cisco Nexus 7000 Series NX-OS Multicast Routing Configuration Guide
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