NETGEAR M7100, M4100 manual Configuring and Using PIM-DM

Port

1/0/13

Switch A

Port

1/0/1

Port

1/0/21

Switch D

Port

1/0/24

Subnet 192.168.4.0/24 Subnet 192.168.2.0/24 Subnet 192.168.1.0/24

Source

IP 192.168.1.1

Port 1/0/9 Port 1/0/10 Subnet 192.168.3.0/24

Subnet 192.168.6.0/24

Port 1/0/22 Port 1/0/22

Subnet 192.168.5.0/24

ProSafe M4100 and M7100 Managed Switches

Switch B

Port

1/0/11

Port

1/0/21

Switch C

Host

IP 192.168.4.2

Figure 45. 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

436 Chapter 28. PIM