3Com Switch 4800G PWR 24-Port, Руководство по настройке

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2) 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.
z
If the result of the RPF check shows that the RPF interface is the incoming interface of the existing
(S, G) entry, this means that the (S, G) entry is correct but the packet arrived from a wrong path and
is to be discarded.
z
If the result of the RPF check shows that the RPF interface is not the incoming interface of the
existing (S, G) entry, this means that the (S, G) entry is no longer valid. The router replaces the
incoming interface of the (S, G) entry with the interface on which the packet actually arrived and
forwards the packet to all the outgoing interfaces.
3) If no corresponding (S, G) entry exists in the multicast forwarding table, the packet is also subject to
an RPF check. The router creates an (S, G) entry based on the relevant routing information and
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 1-7 . Multicast packets travel along
the SPT from the multicast source to the receivers.
Figure 1-7 RPF check process