PIM
557
Managed Switches
Figure 57. 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 that use 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 prune messages, PIM-DM uses two other types of messages:
graft messages and assert messages. 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
Subnet 192.168.1.0/24
Source
Port
Port 1/0/9
Port
Subnet 192.168.3.0/24
IP 192.168.1.1
1/0/13
1/0/1
Subnet 192.168.6.0/24
Subnet 192.168.2.0/24
Subnet 192.168.5.0/24
Subnet 192.168.4.0/24
Host
IP 192.168.4.2
Port 1/0/10
Port
1/0/11
Port
1/0/21
Port 1/0/22
Switch A
Switch D
Switch B
Switch C
Port 1/0/22
Port
1/0/24
Port
1/0/21
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