H3C Technologies H3C SecPath F1000-E - Manual (Page 17)

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• If the interface on which the packet actually arrived is not the RPF interface, the RPF check fails and
the router discards the packet.
2. If the corresponding (S, G) entry exists, and the interface on which the packet actually arrived is
the incoming interface, the router forwards the packet to all the outgoing interfaces.
3. If the corresponding (S, G) entry exists, but the interface on which the packet actually arrived is not
the incoming interface in the multicast forwarding table, the multicast packet is subject to an RPF
check.
• If the RPF interface is the incoming interface of the (S, G) entry, this means the (S, G) entry is correct
but the packet arrived from a wrong path. The packet is to be discarded.
• If the RPF interface is not the incoming interface, this means the (S, G) entry has expired, and router
replaces the incoming interface with the RPF interface. If the interface on which the packet arrived
is the RPF interface, the router forwards the packet to all the outgoing interfaces; otherwise it
discards the packet.
Assume that unicast routes are available in the network, MBGP is not configured, and no multicast static
routes have been configured on Router C, as shown in Figure 1 . Multicast packets travel along the SPT
from the multicast source to the receivers. The multicast forwarding table on Router C contains the (S, G)
entry, with POS 5/1 as the incoming interface.
Figure 1 RPF check process
• When a multicast packet arrives on interface POS 5/1 of Router C, because the interface is the
incoming interface of the (S, G) entry, the router forwards the packet to all outgoing interfaces.
• When a multicast packet arrives on interface POS 5/0 of Router C, because the interface is not the
incoming interface of the (S, G) entry, the router performs an RPF check on the packet: The router
searches its unicast routing table and finds that the outgoing interface to Source (the RPF interface)
is POS 5/1. This means the (S, G) entry is correct and packet arrived along a wrong path. The RPF
check fails and the packet is discarded.
Multicast Static Routes
A multicast static route is an important basis for RPF check. Depending on the application environment,
a multicast static route has the following two functions: