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44.1 PIM-DM6
44.1.1 Introduction to PIM-DM6
PIM-DM6 （ P rotocol Independent Multicast, Dense Mode） is the IP v6 version of Protocol Independent Multicast
Dens e 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 net work environment. There is no difference
compared with the IP v4 version PIM-DM except that the addresses it uses are IP v6 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 IP v6 version PIM-DM.
As a result of continuous development of IP v6 network, it has the network environment of nonsupport IP v6
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 nons upport IP v6 multicast network by single
cast packet of IP v4 encapsulation.
The working process of PIM-DM can be summarized as: Neighbor Discovery, Flooding-P rune, 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-P rune
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 rec eiving the multicast packet, the router will
make RPF examination first according to the unicast table. If the check passes, the router will creat e a (S, G)
table item and forward the multicast packet to all downstream PIM -DM nodes (Flooding). If the RP F
examination fails, i.e. the multicast packet is inputted from the incorrect interfac e, and t hen the message is
discarded. After this procedure, every node will create an (S, G) item in the P IM-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 delet ed from the out put interface list corresponding with the
multicast-forwarding item (S, G). Through this process, a SPT (Shortest Path Tree) is established wit h
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; otherwis e
Chapter 44 IPv6 Multicast Protocol
44-1
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