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Chapter 9

Multicast

whether there are group members on the router or not. This is why it is con-
sidered flooding.

When multiple, equal-cost links exist, the router with the highest IP

address is elected to be the incoming interface (used for RPF). Every router
runs the RPF when it receives the multicast data.

Figure 9.17 depicts the initial multicast flooding in a PIM DM network.

You can see that the data is forwarded to every PIM neighbor throughout the
network. Once a PIM neighbor does the RPF calculation, the router will then
forward the data to interfaces that have active members of the group.

F I G U R E 9 . 1 7

PIM DM flooding

Pruning

After the initial flooding through the PIM neighbors, pruning starts. Pruning
is the act of trimming down the SPT. Because the data has been forwarded
to every router, regardless of group membership, the routers must now
prune back the distribution of the multicast data to routers that actually have
active group members connected.

Figure 9.18 shows the pruning action that occurs for the PIM DM routers

that don't have active group members. Router5 does not have any active
group members, so it sends a prune message to Router3. Even though

HostA

HostF

HostB

HostC

HostD HostE

Source for

224.2.125.125

Member
224.2.125.125

Member

224.2.125.125

Router1

Router2

Router3

HostG

Router5

Router4

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