Rpf Mechanism - H3C S7500E Series Operation Manual

Operation Manual – Multicast
H3C S7500E Series Ethernet Switches

7.1.2 RPF Mechanism

When creating multicast routing table entries, a multicast routing protocol uses the
reverse path forwarding (RPF) mechanism to ensure multicast data delivery along the
correct path.
The RPF mechanism enables routers to correctly forward multicast packets based on
the multicast route configuration. In addition, the RPF mechanism also helps avoid data
loops caused by various reasons.
I. 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
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.
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interface of the existing (S, G) entry, this means that the (S, G) entry is no longer
valid.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.
II. 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
Chapter 7 Multicast Routing and Forwarding
7-2
Configuration