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Operation Manual – Multicast

H3C S3600 Series Ethernet Switches-Release 1510 Chapter 7 PIM Configuration

7-2

7.1.2 Work Mechanism of PIM-DM

The working procedure of PIM-DM is summarized as follows:

z Neighbor discovery

z SPT establishing

z Graft

z RPF check

z Assert mechanism

I. Neighbor discovery

In a PIM-DM network, a multicast router needs to use Hello messages to perform

neighbor discovery and maintain the neighbor relation when it is started. All routers

keep in touch with each other by sending Hello messages periodically, and thus SPT is

established and maintained.

II. SPT establishment

The procedure of establishing SPT is also called Flooding & Prune.

The procedure is as follows:

z PIM-DM assumes that all hosts on the network are ready to receive multicast data.

z When a multicast router receives a multicast packet sent from a multicast source

"S" to a multicast group "G", it begins with an RPF check according to the unicast

routing table.

z If the RPF check passes, the router will create an entry (S, G) and forward the

packet to all the downstream PIM-DM nodes. This process is known as flooding.

z If the RPF check fails, the router considers that the multicast packets travel into the

router through incorrect interfaces and just discards the packets.

After this process is complete, the router creates a (S, G) entry for every host in the

PIM-DM domain.

If there is no multicast group member in downstream nodes, the router sends a prune

message to upstream nodes to inform them not to forward data any more. The

upstream nodes, as informed, remove the related interface from the outgoing interface

list corresponding to the multicast forwarding entry (S, G). The pruning process

continues until there are only necessary branches in PIM-DM. In this way, an SPT

(Shortest Path Tree) rooted at source S is established.

The pruning process is initiated by leaf routers. As shown in

Figure 7-1, the routers

without receivers (such as the router connected to User A) initiate the pruning process

automatically.

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