H3C s3100-52P Operation Manual page 16

Operation Manual – Multicast
H3C S3100-52P Ethernet Switch
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 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.
Assume that unicast routes exist in the network, as shown in
packets travel along the SPT from the multicast source to the receivers.
Source
192.168.0.1/24
IP Routing Table on Switch C
Destination/Mask
192.168.0.0/24
Figure 1-7 RPF check process
A multicast packet from Source arrives to VLAN-interface 1 of Switch C, and the
corresponding forwarding entry does not exist in the multicast forwarding table of
Switch C. Switch C performs an RPF check, and finds in its unicast routing table
that the outgoing interface to 192.168.0.0/24 is VLAN-interface 2. This means that
the interface on which the packet actually arrived is not the RPF interface. The
RPF check fails and the packet is discarded.
A multicast packet from Source arrives to VLAN-interface 2 of Switch C, and the
corresponding forwarding entry does not exist in the multicast forwarding table of
Switch C. The router performs an RPF check, and finds in its unicast routing table
that the outgoing interface to 192.168.0.0/24 is the interface on which the packet
actually arrived. The RPF check succeeds and the packet is forwarded.
Multicast packets
Interface
Vlan-int2
Switch B
Vlan-int2
Vlan-int1
Router A
Vlan-int1
Vlan-int2
Switch C
1-14
Chapter 1 Multicast Overview
Figure 1-7. Multicast
Receiver
Receiver