Pim-Dm; Introduction To Pim-Dm - Accton Technology ES4710BD User Manual

EES4710BD 10 Slots L2/L3/L4 Chassis Switch

19.3 PIM-DM

19.3.1 Introduction to PIM-DM

PIM-DM (Protocol Independent Multicast，Dense Mode) is a dense mode multicast protocol. It is
good for use in small networks as the multicast group members are relatively concentrated in such
network environments.
The work process of PIM-DM can be summarized as the following phases: neighbor discovery,
flooding & prune, grafting.
1. Neighbor discovery
PIM-DM routers need discover neighbors with HELLO packets on start up. Network nodes running
PIM-DM keeps contact with HELLO packets. The HELLO packets are sent in regular intervals.
2. Flooding and Prune
PIM-DM assumes all hosts in the network are ready for receiving multicast data. When a multicast
source S starts sending data to multicast group G, the router will first perform RPF check against the
unicast route table to the multicast packet. If check is ok, the router will create a (S, G) entry and
forward the multicast packet to all downstream PIM-DM nodes in the network (Flooding). If RPF
check fails, indicating the multicast packet is coming from the wrong interface, the packet will be
discarded. After this process, each node in the PIM-DM multicast domain will create a (S, G) entry.
If no multicast group member exists in the downstream nodes, then a prune message will be sent to
the 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 times out, the router restarts 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 routers (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
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