3Com Switch 8800 Advanced Software V5 Configuration Guide

516 CHAPTER 39: IPV6 MULTICAST ROUTING AND FORWARDING CONFIGURATION
The RPF mechanism enables routers to correctly forward IPv6 multicast packets
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 an IPv6 multicast packet sent from a multicast source S to an IPv6
multicast group G, the device first searches its IPv6 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 IPv6 multicast forwarding table,
the device 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 IPv6 multicast forwarding
table, the 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interface of the existing (S, G) entry, this means that the (S, G) entry is no
longer valid.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 device 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 device 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 IPv6 multicast forwarding table.
■ If the interface on which the packet actually arrived is the RPF interface, the RPF
check is successful and the device 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 device discards the packet.
RPF Check
The basis for an RPF check is an IPv6 unicast route. The IPv6 unicast routing table
contains the shortest path to each destination subnet. A multicast routing protocol
does not independently maintain any type of unicast routes; instead, it relies on
the existing unicast routing information in creating multicast routing entries.
When performing an RPF check, the device searches its IPv6 unicast routing table
using the IP address of the "packet source" as the destination address and
automatically selects the optimal route as the RPF route. The outgoing interface in
the corresponding routing entry is the RPF interface and the next hop is the RPF
neighbor. The device considers the path along which the IPv6 multicast packet
from the RPF neighbor arrived on the RPF interface to be the shortest path that
leads back to the source.
n
The above-mentioned "packet source" can mean different things in different
situations: