Planet XGS3-24042 User Manual download pdf (Page 457)

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Chapter 49 IPv6 Multicast Protocol

49.1 PIM-DM6

49.1.1 Introduction to PIM-DM6

PIM-DM6（Protocol Independent Multicast, Dense Mode）is the IPv6 version of Protocol Independent Multicast

Dense Mode. It is a Multicast Routing Protocol in dense mode which adapted to small network. The members

of multicast group are relatively dense under this kind of network environment. There is no difference

compared with the IPv4 version PIM-DM except that the addresses it uses are IPv6 addresses. Thus we don’t

differentiate between PIM-DM and PIM-DM6 in this chapter. All PIM-DM in the text without specific

explanation refers to IPv6 version PIM-DM.

As a result of continuous development of IPv6 network, it has the network environment of nonsupport IPv6

multicast sometimes, so it needs to do the IPv6 multicast operation by tunnel. Therefore, our PIM-DM6

supports configuration on configure tunnel, and passes through nonsupport IPv6 multicast network by single

cast packet of IPv4 encapsulation.

The working process of PIM-DM can be summarized as: Neighbor Discovery, Flooding-Prune, and Graft.

1. Neigh hour Discovery

When PIM-DM router is started at beginning, Hello message is required to discover neighbors. The network

nodes running PIM-DM use Hello message to contact each other. PIM-DM Hello message is sent

periodically.

2. Flooding-Prune

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

source S begins to send data to a multicast group G, after receiving the multicast packet, the router will

make RPF examination first according to the unicast table. If the check passes, the router will create a (S, G)

table item and forward the multicast packet to all downstream PIM-DM nodes (Flooding). If the RPF

examination fails, i.e. the multicast packet is inputted from the incorrect interface, and then the message is

discarded. After this procedure, every node will create an (S, G) item in the PIM-DM multicast domain. If

there is no multicast group member in the downstream nodes, then a Prune message is sent to upstream

nodes notifying not to forward data to this multicast group any more. After receiving Prune message, the

corresponding interfaces will be deleted from the output interface list corresponding with the

multicast-forwarding item (S, G). Through this process, a SPT (Shortest Path Tree) is established with

source S as root. Prune process is started by a sub-router.

The process above is called Flooding-Prune process. Each pruned node also provides overtime

mechanism at the same time. In case of overtime of prune, the router will restart flooding-prune process.

Flooding-prune of PIM-DM is conducted periodically

3. RPF examination

Adopting RPF examination, PIM-DM establishes a multicast forwarding tree initiating from data source,

using existing unicast routing table. When a multicast packet arrives, the router will determine the

correctness of its coming path first. If the arrival interface is the interface connected to multicast source

indicated by unicast routing, then this multicast packet is considered to be from the correct path; otherwise

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