Implementation Of The Rpf Mechanism; Rpf Check - H3C s3100-52P Operation Manual

Operation Manual – Multicast
H3C S3100-52P Ethernet Switch
To process the same multicast information from different peers received on
different interfaces of the same device, every multicast packet is subject to a
Reverse Path Forwarding (RPF) check on the incoming interface. The result of the
RPF check determines whether the packet will be forwarded or discarded. The
RPF check mechanism is the basis for most multicast routing protocols to
implement multicast forwarding.
The RPF mechanism enables multicast devices to forward multicast packets correctly
based on the multicast route configuration. In addition, the RPF mechanism also helps
avoid data loops caused by various reasons.

1.4.1 Implementation of the RPF Mechanism

Upon receiving a multicast packet that a multicast source S sends to a multicast group
G, the multicast device 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
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
interface of the existing (S, G) entry, this means that the (S, G) entry is no longer
valid.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
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
RPF check fails and the router discards the packet.

1.4.2 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
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Chapter 1 Multicast Overview