Netgear GSM7328SNA 7000 Series Managed Switch Administration Guide for Softwar - Page 374
Configuring and Using PIM-DM
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Subnet 192.168.4.0/24 Subnet 192.168.2.0/24 Subnet 192.168.1.0/24 Subnet 192.168.5.0/24 Source IP 192.168.1.1 ProSafe 7000 Managed Switch Release 8.0.3 Switch A Port 1/0/13 Port 1/0/1 Port 1/0/9 Port 1/0/10 Subnet 192.168.3.0/24 Port 1/0/11 Switch B Switch D Port 1/0/21 Port 1/0/24 Subnet 192.168.6.0/24 Port 1/0/22 Port 1/0/22 Port 1/0/21 Switch C Host IP 192.168.4.2 Figure 1. Configuring and Using PIM-DM PIM-DM uses the existing unicast routing table and join, prune, and graft mechanism to build a tree. PIM-DM creates source-based shortest-path distribution trees making use of reverse path forwarding (RPF). PIM-DM cannot be used to build a shared distribution tree, as PIM-SM can. PIM-DM assumes that when a sender starts sending data, all downstream routers and hosts want to receive a multicast datagram. PIM-DM initially floods multicast traffic throughout the network. Routers that do not have any downstream neighbors prune back the unwanted traffic. Apart from the prune messages, PIM-DM makes use of two more messages: graft and assert. Graft messages are used whenever a new host wants to join the group. Assert messages are used to shut off duplicate flows onto the same multi-access network. To minimize the repeated flooding of datagrams and subsequent pruning associated with a particular (S,G) pair, PIM-DM uses a state refresh message. This message is sent by the routers directly connected to the source and is propagated throughout the network. When 374 | Chapter 26. PIM