Information Technology Reference
In-Depth Information
Dense Mode
When operating in dense mode, the PIM protocol uses a “push” methodology. Multicast traffic
is flooded out all PIM-DM adjacencies every 3 minutes, creating state for every (S,G) in every
router in the network. Unwanted traffic is pruned. This process repeats every 3 minutes.
The configuration to enable PIM-DM is minimal. Multicast routing must be enabled on
every node in the network using the global configuration command
ip multicast-routing
.
Every interface in the network must have PIM-DM enabled to allow PIM adjacencies to
form. The purpose of PIM adjacencies is very similar to IGP adjacencies, in that they define
the available interfaces on which MDTs can be built. This is enabled with the interface
configuration command
ip pim dense-mode
.
Another command also enables PIM-DM, but with the addition of an RP, the network
converts to Sparse Mode (PIM-SM). This interface configuration command is
ip pim
sparse-dense-mode
.
It is recommended that you enable PIM Sparse/Dense Mode rather than just PIM-DM,
thereby allowing an RP to be configured to migrate the network to PIM-SM. With PIM-
DM, traffic is forwarded down source trees. The RPF function is performed against the
source address only, because a PIM-DM network has no shared root.
PIM-DM Example
This section describes the operation of a PIM-DM network. Figure 11-7 shows the net-
work's initial topology. The focus of this example is at a high level. It doesn't delve too
deeply into the state details.
Figure 11-7
Initial Topology for the PIM-DM Network
R3
S2
S3
R1
R6
S3
S2
S2
S2
S4
S2
E0
Source
R2
Receiver
2
E0
R5
S3
S4
S3
S4
S5
S3
R4
S2
S2
R7
E0
Receiver
1
