3Com MSR 50 Series Configuration Manual page 1098

1098 C 68: M
HAPTER ULTICAST
R F C
OUTING AND ORWARDING
Implementation of the RPF mechanism
Upon receiving a multicast packet that a multicast source S sends to a multicast
group G, the router first searches its multicast forwarding table:
1 If the corresponding (S, G) entry exists, and the interface on which the packet
actually arrived is the incoming interface in the multicast forwarding table, the
router forwards the packet to all the outgoing interfaces.
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.
If the result of the RPF check shows that the RPF interface is the incoming
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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.
If the result of the RPF check shows that the RPF interface is not the incoming
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interface of the existing (S, G) entry, this means that the (S, G) entry is no
longer valid.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.
If the interface on which the packet actually arrived is the RPF interface, the RPF
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check is successful and the router forwards the packet to all the outgoing
interfaces.
If the interface on which the packet actually arrived is not the RPF interface, the
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RPF check fails and the router discards the packet.
RPF check
The basis for an RPF check is a unicast route or a multicast static route. A unicast
routing table contains the shortest path to each destination subnet, while a
multicast static routing table lists the RPF routing information defined by the user
through static configuration. A multicast routing protocol does not independently
maintain any type of unicast route; instead, it relies on the existing unicast routing
information or multicast static routes in creating multicast routing entries.
When performing an RPF check, a router searches its unicast routing table and
multicast static routing table at the same time. The specific process is as follows:
1 The router first chooses an optimal route from the unicast routing table and
multicast static routing table:
The router automatically chooses an optimal unicast route by searching its
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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.
The router automatically chooses an optimal multicast static route by searching
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its multicast static routing table, using the IP address of the "packet source" as
ONFIGURATION