Implementation of the rpf mechanism, Rpf check – H3C Technologies H3C S3600 Series Switches User Manual

Page 386

1-12

In the network, multicast packet transmission is based on the guidance of the multicast forwarding

table derived from the unicast routing table or the multicast routing table specially provided for

multicast.

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.

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existing (S, G) entry, this means that the (S, G) entry is no longer valid.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.

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

This manual is related to the following products: