Accton Technology ES4710BD manual Pim-Dm, Introduction to PIM-DM

ES4710BD 10 Slots L2/L3/L4 Chassis Switch

19.3PIM-DM

19.3.1Introduction 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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