3Com Switch 4800G 24-Port Configuration Manual page 702

702 C 49: 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.
3 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. The packet is to be discarded.
4 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interface of the existing (S, G) entry, this means that the (S, G) entry is no longer
valid.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.
5 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.
6 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.
7 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 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:
2 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.
3 The router automatically chooses an optimal multicast static route by searching its
multicast static routing table, using the IP address of the "packet source" as the
ONFIGURATION