H3C Technologies H3C S3100 Series Switches - Manual (Page 286)

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using the RPF interface as the incoming interface, and installs the entry into the multicast
forwarding table.
z If the interface on which the packet actually arrived is the RPF interface, the RPF check is
successful and the router forwards the packet to all the outgoing interfaces.
z If the interface on which the packet actually arrived is not the RPF interface, the RPF check fails
and the router discards the packet.
RPF Check
The basis for an RPF check is a unicast route. A unicast routing table contains the shortest path to each
destination subnet. A multicast routing protocol does not independently maintain any type of unicast
route; instead, it relies on the existing unicast routing information in creating multicast routing entries.
When performing an RPF check, a router searches its unicast routing table. The specific process is as
follows: The router automatically chooses an optimal unicast route by searching its unicast routing table,
using the IP address of the “packet source” as the destination address. The outgoing interface in the
corresponding routing entry is the RPF interface and the next hop is the RPF neighbor. The router
considers the path along which the packet from the RPF neighbor arrived on the RPF interface to be the
shortest path that leads back to the source.
Assume that unicast routes exist in the network, as shown in Figure 1-10 . Multicast packets travel along
the SPT from the multicast source to the receivers.
Figure 1-10 RPF check process
Source
192.168.0.1/24
Receiver
Receiver
Router A
Switch B
Switch C
Vlan-int2
Vlan-int1
Vlan-int1
Vlan-int2
Multicast packets
Destination/Mask
IP Routing Table on Switch C
192.168.0.0/24
Interface
Vlan-int2
z A multicast packet from Source arrives to VLAN-interface 1 of Switch C, and the corresponding
forwarding entry does not exist in the multicast forwarding table of Switch C. Switch C performs an
RPF check, and finds in its unicast routing table that the outgoing interface to 192.168.0.0/24 is
VLAN-interface 2. This means that the interface on which the packet actually arrived is not the RPF
interface. The RPF check fails and the packet is discarded.
z A multicast packet from Source arrives to VLAN-interface 2 of Switch C, and the corresponding
forwarding entry does not exist in the multicast forwarding table of Switch C. The router performs
an RPF check, and finds in its unicast routing table that the outgoing interface to 192.168.0.0/24 is