74
•
If a match is found but the receiving interface is not the incoming interface of the entry, the
router determines the RPF route back to the packet source. Then, it makes the following
judgments:
{
If the RPF interface of the RPF route is the incoming interface, it means that the forwarding
entry is correct but the packet traveled along a wrong path. The router discards the packet.
{
If the RPF interface is not the incoming interface, it means that the forwarding entry has
expired. The router replaces the incoming interface with the RPF interface and matches the
receiving interface against the RPF interface. If the receiving interface is the RPF interface,
the router forwards the packet to all outgoing interfaces. Otherwise, it discards the packet.
As shown in
, unicast routes are available in the network, MBGP is not configured, and no
static multicast routes have been configured on Switch C. Multicast packets travel along the SPT
from the multicast source to the receivers. The multicast forwarding table on Switch C contains the
(S, G) entry, with VLAN-interface 20 as the incoming interface.
Figure 26 RPF check process
•
If a multicast packet arrives at Switch C on VLAN-interface 20, Switch C forwards the packet to
all outgoing interfaces.
•
If a multicast packet arrives at VLAN-interface 10, Switch C performs an RPF check on the
packet. Switch C searches its unicast routing table and finds that the outgoing interface to the
source (the RPF interface) is VLAN-interface 20. It means that the (S, G) entry is correct but the
packet traveled along a wrong path. The packet fails the RPF check, and Switch C discards the
packet.
Static multicast routes
A static multicast route is an important basis for RPF check. Depending on the application
environment, a static multicast route can change an RPF route and create an RPF route.
Changing an RPF route
Typically, the topology structure of a multicast network is the same as that of a unicast network, and
multicast traffic follows the same transmission path as unicast traffic does. You can configure a static
multicast route for a multicast source to change the RPF route. This allows the router to create a
transmission path for multicast traffic that is different from the transmission path for unicast traffic.
Source
192.168.0.1/24
Receiver
Receiver
Switch A
Switch B
Switch C
Vlan-int20
Vlan-int10
Vlan-int10
Multicast packets
Destination/Mask
IP Routing Table on Switch C
192.168.0.0/24
Interface
Vlan-int20
