PIM DM

PIM DM
There are three types of Protocol Independent Multicast (PIM): sparse mode, dense mode, and
a combination of the two. Although PIM dense mode (PIM DM) maintains several functions,
the ones that are discussed here are flooding, pruning, and grafting. We’ll talk about sparse
mode later in this chapter.
PIM is considered “protocol independent” because it actually uses the unicast route table for
RPF and multicast forwarding. PIM DM understands classless subnet masking and uses it when
the router is running an IP classless unicast protocol.
PIM DM routers establish neighbor relationships with other routers running PIM DM. It
uses these neighbors to establish an SPT and forward multicast data throughout the network.
The SPT created by PIM DM is based on source tree distribution.
PIM, either sparse mode or dense mode, is the method that Cisco recommends
for multicast routing on their routers.
Flooding When a multicast source begins to transmit data, PIM runs the RPF, using the unicast
route table to verify that the interface leads toward the source. It then forwards the data to
all PIM neighbors. Those PIM neighbors then forward the data to their PIM neighbors. This
happens throughout the network, whether there are group members on the router or not. Every
multicast-enabled router participates; that is why it is considered flooding and is where the term
“dense mode” comes from.
614 Chapter 19  Understanding and Configuring Multicast Operation
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 19.17 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. After a PIM neighbor does the RPF
calculation, the router then forwards the data to interfaces that have active members of the group.
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 19.18 shows the pruning action that occurs for the PIM DM routers that don’t have
active group members. Router 5 does not have any active group members, so it sends a prune
message to Router 3. Even though Router 4 has a network that does not have members, it has
an interface that does, so it will not send a prune message.
Four criteria merit a prune message being sent by a router:
 The incoming interface fails the RPF check.
 There are no directly connected active group members and no PIM neighbors. (This is
considered a leaf router because it has no downstream PIM neighbors.)
 A point-to-point non-leaf router receives a prune request from a neighbor.
 A LAN non-leaf router receives a prune request from another router, and no other
router on the segment overrides the prune request.
If any of these criteria are met, a prune request is sent to the PIM neighbor and the SPT is
pruned back.