3Com Switch 8800 Advanced Software V5 Configuration Guide

504 CHAPTER 38: MULTICAST ROUTING AND FORWARDING CONFIGURATION
Implementation of the RPF mechanism
Upon receiving a multicast packet that a multicast source S sends to a multicast
group G, the 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
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 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interface of the existing (S, G) entry, this means that the (S, G) entry is no
longer valid.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 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 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 a unicast route or a multicast static route. A unicast
routing table contains the shortest path to each destination address, 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, the device searches its unicast routing table and
multicast static routing table at the same time. The specific process is as follows:
1 The device first chooses an optimal route from the unicast routing table and
multicast static routing table:
■ The device 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 device 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.
■ The device automatically chooses an optimal multicast static route by searching
its multicast static routing table, using the IP address of the "packet source" as