196 CHAPTER 15: MULTICAST OVERVIEW
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 59. Multicast
packets travel along the SPT from the multicast source to the receivers.
Figure 61 RPF check process
■ 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.
■ 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 the interface on
which the packet actually arrived. The RPF check succeeds and the packet is
forwarded.
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
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