Protocol Independent Multicast Sparse Mode-Dense Mode (PIM-SMDM) is a protocol designed to manage both sparse and dense multicast groups, efficiently handling varying multicast distribution patterns. In dense mode, it assumes listeners on all subnetworks, initially flooding the network and then pruning back areas without listeners. In sparse mode, it assumes few listeners and forwards traffic only to known listeners, reducing unnecessary transmission. PIM-SMDM switches between modes based on the multicast group's status, treating interfaces accordingly. A group is sparse if the router knows about a Rendezvous Point (RP) for it. Its adaptability makes it a versatile solution for diverse network scenarios.

Protocol Independent Multicast Sparse Mode-Dense Mode (PIM-SMDM) manages both sparse and dense multicast groups, seamlessly switching modes based on the group's status. In dense mode, it floods the network with multicast traffic and prunes areas without listeners. In sparse mode, it forwards traffic only to known listeners, reducing unnecessary data transmission. The protocol treats interfaces as dense or sparse based on the group's mode, considering a group sparse if a Rendezvous Point (RP) is known. PIM-SMDM efficiently distributes multicast streams, optimizes network resources, and adapts to different multicast group modes, making it suitable for diverse network scenarios.

The required steps to configure PIM-SMDM are the following:

All multicast group states are dynamically maintained as the result of IGMP Report/Leave and PIM Join/Prune messages.This section provides the steps to configure the PIM-SMDM feature. Configuration steps and examples are used for two relevant scenarios. The following figure displays the network topology used in these examples.

There are two topologies for this sparse-dense mode configuration. Understanding these modes helps network administrators select the best multicast strategy for their network, ensuring efficient and reliable traffic delivery.

The network topology in Figure 1-1, includes several routers and hosts within a multicast network. Key components are Router_C, the Rendezvous Point (RP), and Host_1 and Host_2, which join a multicast group. Subnet 1 connects Host_1, Host_2, Router_E, and Router_F, with the latter two managing multicast traffic on the subnet.

In the network topology shown in Figure 1-2, Source_1 (10.10.1.52) sends multicast data to group address 224.0.1.3. Host_1 shows interest in this group by sending an IGMP membership report, which Router_C processes to associate its eth1 interface with the group. As data packets flow from Source_1, each router creates an (S,G) entry in its multicast routing table. Router_C forwards the packets through eth1 to Host_1 and, having a downstream receiver, does not send a prune message to its upstream neighbor, Router_E, ensuring continuous delivery of multicast traffic to interested hosts.

To enable IP multicast routing on all of the PIM routers inside the PIM domain:

Enable PIM-SMDM on all participating interfaces within each of routers inside the PIM domain on which you want to run PIM. In the following sample configuration, both eth1 and eth2 are enabled for PIM-SMDM on the router.

Here is the sample configuration for Router_C:

The show ip pim interface command displays the interface details for Router_C.

The following examples differentiates the group operating in sparse mode versus dense mode:

Every PIM multicast group needs to be associated with the IP address of a Rendezvous Point (RP), which is a router that resides in a multicast network domain. The address of the RP is used as the root of a group-specific distribution tree. All nodes in the domain that want to receive traffic sent to the group are aware of the address of the RP. For all senders to reach all receivers within a group, all routers in the domain must be able to map to the RP address configured for the group. There can be several RPs configured in a network deploying PIM-SM, each serving a different group.

You can statically configure a RP by specifying the RP address in every router in the PIM domain. The use of statically configured RPs is ideal for small network environments or ones that do not require many RPs and/or require changing the assignment of the RPs often. Changing the assignment of an RP requires the re-configuration of the RP address in all of the routers in the PIM domain.

In static RP configurations, RP failover is not available.

When configuring the RP statically, do the following:

The network topology shown in the Figure 1-1, includes several routers, a source, and hosts in different subnets.

Here is the sample configuration for Router_D:

At Router_D, the show ip pim rp mapping command shows that 10.10.1.5 is the RP for all multicast groups 224.0.0.0/4, and is statically configured. All other routers will have a similar output:

At Router_D, use the show ip pim rp-hash command to display the selected RP for a specified group (224.0.1.3):

The show ip pim interface command displays the interface details for Router_E, and shows that Router_E is the Designated Router on Subnet 1.

The show ip pim mroute command displays the IP multicast routing table. In this table, the following fields are defined:

At Router_E, eth2 is the incoming interface of the (*, G) entry, and eth1 is on the outgoing interface list of the
(*, G) entry. This means that there is a group member through eth1, and the RP is reachable through eth2.

The 0 position on this 32-bit index is for eth1 (as illustrated in the interface display above). The j on the 0 index indicates that the Join has come from eth1.

Since Router_C is the RP, and the root of this multicast tree, the show ip pim mroute command on Router_C shows RPF nbr as 0.0.0.0 and RPF idx as none.

For configuring Rendezvous point dynamically refer Configure Rendezvous Point Dynamically Using Bootstrap Router Method and Configuring Rendezvous Point Statically

The network topology described in Figure 1-2, the Source_1 address is 10.10.1.52 and the group address is set to 224.0.1.3.

In this example all routers are running PIM-SMDM.

The network topology shown in the Figure 1-2, includes a source, three routers, and a host in a subnet.

Enter the commands listed in this section to confirm the previous configurations.

The show ip pim mroute command displays the IP multicast routing table.

The show ip pim dense-mode mroute command displays the IP PIM-DM multicast routing table