3Com Switch 8800 Configuration Guide Chapter 28 PIM-DM Configuration
28-2
During this process, PIM-DM uses the RPF check and the existing unicast routing table
to build a multicast forwarding tree rooted at the data source. When a packet arrives,
the router will first judge the correctness of the path. If the interface that the packet
arrives is the one indicated by the unicast routing to the multicast source, the packet is
regarded to be from the correct path. Otherwise, the packet will be discarded as a
redundancy packet without the multicast forwarding. The unicast routing information as
path judgment can come from any unicast routing protocol independent of any
specified unicast routing protocol such as the routing information learned by RIP and
OSPF
III. Assert mechanism As shown in the following figure, both routers A and B on the LAN have their own
receiving paths to multicast source S. In this case, when they receive a multicast packet
sent from multicast source S, they will both forward the packet to the LAN. Multicast
Router C at the downstream node will receive two copies of the same multicast packet.
Receiver
Router A Router B
Router C
Multicast packets forwarded by the
upstream node
Figure 28-1 Assert mechanism diagram
When they detect such a case, routers need to select a unique sender by using the
assert mechanism. Routers will send Assert packets to select the best path. If two or
more than two paths have the same priority and metric, the path with a higher IP
address will be the upstream neighbor of the (S, G) entry, which is responsible for
forwarding the (S, G) multicast packet.
Note:
Currently assert mechanism is not available on the Switch 8800.