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upstream nodes to inform the upstream node that no more forwarding for that multicast
group is necessary. The upstream nodes will delete the corresponding interface, multicast
forwarding entry(S,G), from the outgoing interface list. Hence a shortest path tree (SPT)
rooted by source S is established. The prune process is initiated by leaf routers first.
The above procedures are referred to as the Flooding-Prune process. A timeout
mechanism is provided for each pruned nodes, when the prune timeout, the route restart
the flooding-prune process. The PIM-DM flooding-prune process is performed in regular
intervals.
3. RPF check
PIM-DM employs the RPF check method to build a multicast tree rooted from the data
source according to the existing unicast route table. When a multicast arrives at the router,
its path correctness is checked first. If as indicated by the unicast route, the arriving
interface is the interface to the multicast source, the packet is considered to be from the
correct path; otherwise, the multicast packet is discarded as a redundant packet. The
unicast route information used as the route decision fact is not dependent on specific
unicast routing protocol, but can be the route information of any unicast routing protocols,
such as route discovered by RIP, OSPF, etc.
4. Assert mechanism
If two routes (A and B) in the same LAN segment both have a receiving path to multicast
source S, both will forward the multicast packet sent by multicast source S in the LAN. As
a result, the downstream multicast router C will receive two identical multicast packets. On
detecting such situation, the router will decide a unique forwarder through the Assert
mechanism. The best forwarding path is decided by sending Assert packets. If two or
more paths have the same priority and costs, then the node with a larger IP address is
selected as the upstream neighbor for the (S, G) entry and is responsible for the
forwarding of multicast packet for that (S, G) entry,.
5. Graft
If a pruned downstream node needs to restore to the forwarding state, the node will send
a graft packet to ask the upstream to restore multicast data forwarding.
16.3.2 PIM-DM Configuration
16.3.2.1 PIM-DM Configuration Task Sequence
1、 Enable PIM-DM (required)
2、 Configure PIM-DM sub-parameters (optional)
Configure PIM-DM interface parameters
Configure PIM-DM HELLO packet interval
1. Enable PIM-DM
Basic configuration of PIM-DM routing protocol on route switch is quite simple: just enable