nx.os.and.cisco.nexus.switching.2nd.edition.1587143046

Reverse Path Forwarding
Figure 4-3. Bidirectional Shared Tree for 239.0.0.25
An additional concept beyond multicast distribution trees that is important for multicast is
that of Reverse Path Forwarding (RPF). Multicast, by design, is traffic not intended for
every system on a network but rather sent only to receivers that request it. Routers in the
network must form a path toward the source or RP. The path from the source to the receivers
flows in the reverse direction from which the path was created when the receiver requested
to join the group. Each incoming multicast packet undergoes an RPF check to verify it was
received on an interface leading to the source. If the packet passes the RPF check, it is
forwarded; if not, the packet is discarded. The RPF check is done to minimize the potential
for duplicated packets and maintain source integrity.
Protocol Independent Multicast (PIM)
With a solid understanding of the multicast distribution tree modes and the RPF, the next
concept is Protocol Independent Multicast (PIM), which is an industry-standard protocol
developed to leverage any existing underlying Interior Gateway Routing (IGP) protocol,
such as Enhanced Interior Gateway Routing Protocol (EIGRP), Open Shortest Path First
(OSPF), or Routing Information Protocol (RIP), to determine the path of multicast packets in
the network. PIM does not maintain its own routing table and as such has a much lower
overhead when compared with other multicast routing technologies.