The Protocol Independent Multicasting-Sparse Mode (PIM-SM) is a multicast routing protocol designed to operate efficiently across Wide Area Networks (WANs) with sparsely distributed groups. It helps geographically dispersed network nodes to conserve bandwidth and reduce traffic by simultaneously delivering a single stream of information to multiple locations. PIM-SM uses the IP multicast model of receiver-initiated membership, supporting both shared and shortest-path trees, and uses soft-state mechanisms to adapt to changing network conditions. It relies on a topology-gathering protocol to populate a multicast routing table with routes.

A Rendezvous Point (RP) router is configured as the root of a non-source-specific distribution tree for a multicast group. Join messages from receivers for a group are sent towards the RP. Data from senders is sent to the RP so that receivers can discover who the senders are, and receive traffic destined for the group.

The Multicast Routing Information Base (MRIB) is a multicast topology table derived from the unicast routing table. In PIM-SM, the MRIB decides where to send Join/Prune messages. It also provides routing metrics for destination addresses. These metrics are used when sending and processing Assert messages.

Reverse Path Forwarding (RPF) is an optimized form of flooding, in which the router accepts a packet from SourceA through Interface IF1, only if IF1 is the interface the router uses to reach SourceA. To determine if the interface is correct, it consults its unicast routing tables. The packet that arrives through interface IF1 is forwarded because the routing table lists this interface as the shortest path. The router's unicast routing table determines the shortest path for the multicast packets. Because a router accepts a packet from only one neighbor, it floods the packet only once, meaning that (assuming point-to-point links) each packet is transmitted over each link, once in each direction.

The Tree Information Base (TIB) is a collection of states at a PIM router storing the state of all multicast distribution trees at that router. The TIB is created by receiving Join/Prune messages, Assert messages, and IGMP information from local hosts.

Upstream indicates that traffic is going towards the root of the tree. The root of the tree might be either the Source or the RP.

Downstream indicates that traffic is going away from the root of the tree. The root of tree might be either the Source or the RP.

In Source-Based Trees, the forwarding paths are based on the shortest unicast path to the source. If the unicast routing metric used is hop counts, the branches of the multicast Source-Based Trees are minimum hop. If the metric used is delay, the branches are minimum delay. A corresponding multicast tree directly connects the source to all receivers for every multicast source. All traffic to the members of an associated group passes along the tree made for their source. Source-Based Trees have two entries with a list of outgoing interfaces -- the source address and the multicast group.

Shared trees, or RP trees (RPT), rely on a central router called the Rendezvous Point (RP) that receives all traffic from the sources, and forwards that traffic to the receivers. There is a single tree for each multicast group, regardless of the number of sources. Only the routers on the tree know about the group, and information is sent only to interested receivers. With an RP, receivers have a place to join, even if no source exists. The shared tree is unidirectional, and information flows only from the RP to the receivers. If a host other than the RP has to send data on the tree, the data must first be tunneled to the RP, then multicast to the members. This means that even if a receiver is also a source, it can only use the tree to receive packets from the RP, and not to send packets to the RP (unless the source is located between the RP and the receivers).

When a new multicast sender starts sending data packets, or a new receiver starts sending Join messages towards the RP for that multicast group, the sender needs to know the next-hop router towards the RP. The bootstrap router (BSR) provides group-to-RP mapping information to all the PIM routers in a domain, allowing them to map to the correct RP address.

Protocol Independent Multicast - Sparse Mode (PIM-SM) is a multicast routing protocol that uses an explicit join model to deliver data only to network segments that have active receivers. Unlike dense mode protocols that flood traffic, PIM-SM is designed for environments where group members are sparsely distributed across the network. It relies on a central point known as the Rendezvous Point (RP) to coordinate the delivery of multicast traffic from sources to receivers.

PIM-SM constructs a unidirectional shared tree (RPT) rooted at the RP to distribute traffic from sources to receivers. When a source begins sending data, its local first-hop router encapsulates the data in Register messages and sends them to the RP. Receivers join the shared tree by sending Join messages toward the RP. PIM-SM also supports the transition to a Shortest Path Tree (SPT) for more efficient data delivery directly from the source to the receiver. The protocol operates independently of the underlying unicast routing protocol, though it relies on the unicast routing table for Reverse Path Forwarding (RPF) checks.

Before configuring PIM-SM, ensure the following foundational network settings are in place:

The topology consists of three primary OcNOS routers (Router-1, Router-2, and Router-3) forming a multicast domain.

The following PIM Sparse Mode configurations apply to all PIM routers within the OcNOS multicast domain:

To verify the PIM-SM configuration and operation, use the following show commands.

Verify PIM Interface Status. Check that PIM is active on the intended interfaces and verify the Designated Router (DR) election.

Verify PIM Neighbors

Ensure that PIM adjacency is established between neighboring routers to allow multicast control traffic flow.

Verify RP Mappings Confirm that the router correctly identifies the static RP for the multicast group ranges.

Verify Multicast Routing Table

The mroute table shows the active multicast entries, including the incoming interface (IIF) and outgoing interface list (OIL).

Note: At Router-2, the show ipv6 pim rp mapping command shows that 26.0.0.5 is the RP for all multicast groups in the range 224.0.0.0/4, and it is statically configured. All other routers display similar output:

Use Case: Enterprise Video Distribution An organization needs to stream high-definition training videos to multiple offices without saturating the network.

Solution: Implementing PIM-SM allows the video source to send a single stream to the RP. Only routers with employees actively watching the stream will join the shared tree and receive the traffic, ensuring efficient bandwidth utilization across the provider backbone.

In a Protocol Independent Multicast Sparse Mode (PIM-SM) domain, each multicast group must map to a single Rendezvous Point (RP). The RP acts as the root of the group-specific shared distribution tree (RPT). All routers within the PIM domain must know the RP address to correctly forward multicast Join and Register messages.

 

When you configure the RP statically, you manually define the RP IP address on every router in the PIM domain. Each router must maintain an identical RP-to-group mapping to ensure end-to-end multicast reachability.

When configuring the RP statically, do the following:

Using the topology depicted in Figure 1-4, Router_C is the RP, and all routers are statically configured with RP information. Host_1 and Host_2 join group 224.0.1.3 for all the sources. They send the IGMP membership report to Subnet 1. Two routers are attached to Subnet 1, Router_E and Router_F; both have default DR priority on eth1. Since Router_E has a higher IP address on interface eth1, it becomes the Designated Router, and is responsible for sending Join messages to the RP (Router_C).

For configurations refer PIM Sparse Mode Configuration.

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

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:

Override RP cnt: 0At Router_D, use the show ip pim rp-hash command to display the selected RP for a specified group (224.0.1.3):

Override RP cnt: 0At Router_D, use the show ipv6 pim rp-hash command to display the selected RP for a specified group (ff02::1).

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:

The show ipv6 pim mroute command displays the IPv6 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.

Small Enterprise Multicast Deployment:

Scenario

An enterprise network deploys Protocol Independent Multicast Sparse Mode to support internal video distribution and software update streaming.

The multicast domain contains 8 routers and fewer than 10 multicast groups.

The Bootstrap Router (BSR) mechanism is a dynamic method for configuring Rendezvous Points (RP) within a Protocol Independent Multicast (PIM) Sparse Mode domain. It eliminates the need for manual, static RP assignments on every router, which is particularly critical for large or complex multicast networks. The BSR acts as a central distribution point, ensuring all routers in the domain learn the group-to-RP mappings necessary for multicast traffic forwarding.

In a PIM domain, the BSR mechanism utilizes a set of configured Candidate-BSRs (C-BSRs) and Candidate-RPs (C-RPs). Through an election process, the C-BSR with the highest priority value is elected as the BSR for the domain.

Once elected, the BSR performs the following technical functions:

Before configuring BSR, ensure that the underlying Interior Gateway Protocol (IGP) such as OSPF or IS-IS is fully operational to provide reachability. Multicast routing must be enabled globally on all participating nodes.

BSR configurations apply to PIM-enabled routers within the Service Provider MPLS or IP network.

The PIM Bootstrap Router (BSR) topology is designed to support dynamic Rendezvous Point (RP) discovery across a multicast domain. It consists of five key OcNOS routers interconnected via physical board interfaces to manage multicast traffic flows between sources and receivers.

The following routers form the BSR domain, each performing a specific multicast function:

Verify the dynamic RP and BSR status using the following show commands.

Verify BSR Information: Check which router has been elected as the BSR and its current priority.

For IPV6

Verify Group-to-RP Mappings: Ensure the routers have learned the RP for specific multicast groups through the BSR mechanism.

For IPV6

Use Case: Large Enterprise Campus In a campus network with hundreds of VLANs, manually configuring a static RP on every access and distribution switch is error-prone. By implementing BSR, the administrator only needs to configure two core switches as C-BSRs and C-RPs. If the primary core switch fails, the BSR mechanism automatically promotes the secondary RP, ensuring uninterrupted video streaming or software distribution services.

Protocol Independent Multicast - Sparse Mode (PIM-SM) Group-to-Rendezvous Point (RP) mapping is a critical mechanism used in Any Source Multicast (ASM) domains to identify the RP for a specific multicast group. Each PIM-SM router maintains these mappings to determine how to join the shared tree for a group or where to send Register messages.

In a PIM-SM domain, routers learn the Group-to-RP mapping through various mechanisms such as Static RP, Auto-RP, or Bootstrap Router (BSR). OcNOS utilizes a defined algorithm to select the best RP from the available Group-to-RP mappings. This selection algorithm is designed to be consistent across the domain and does not prioritize based on the PIM mode or the specific mechanism (for example: Static vs. BSR) through which the mapping was learned.

Before configuring Group-to-RP mappings, ensure the following base configurations are present on all participating routers:

The following configurations should be set up to align with the topology:

Configure various nodes within the topology to set up an PIM-SM Group-to-RP Mapping Scenario.

The topology consists of a linear path from the receiver to the source to demonstrate RP mapping and traffic flow.

The following PIM-SM Group-to-RP Mapping configurations apply to the RP and PIM-capable nodes within the Multicast network.

Here is the sample configuration for LHR:

Verify the PIM neighbor status on the LHR (R1)

Check the learned RP information on the LHR (R1). The output shows the group ranges and the associated RP addresses learned via BSR

 

Confirm multicast traffic is flowing from the incoming interface to the outgoing interface connected to the receiver.

Enterprise Multicast Isolation A corporation needs to separate internal training video traffic from general background multicast.

By configuring two different RPs and using BSR group-lists, the network engineer maps the training range (239.1.1.0/24) to a high-performance RP while keeping general traffic on a default RP. This optimizes traffic flow and provides administrative control over different multicast streams.

PIM Anycast Rendezvous Point (RP) is a networking solution designed to provide redundancy and load sharing for the RP in a Protocol Independent Multicast - Sparse Mode (PIM-SM) domain. By configuring multiple RPs with the same Anycast IP address, the network ensures high availability; if one RP fails, multicast traffic automatically reroutes to the topologically closest RP based on unicast routing metrics.

Before configuring Anycast RP, ensure the following foundational protocols are active on all participating nodes:

The Anycast RP validation network is designed to demonstrate high availability and path optimization for both IPv4 and IPv6 multicast traffic across a service provider fabric. The topology is categorized into the following functional roles:

The following PIM Anycast RP configurations applies to RP nodes and PIM domain routers within the OcNOS network.

Verify the PIM Anycast RP configuration:

Verify RP Mapping and Anycast Members Use the show ip pim rp mapping command on the RP nodes to confirm the Anycast-RP members are correctly identified.

Verify PIM Neighbor States Ensure all routers have established PIM adjacencies over their respective interfaces.

Verify Multicast Routing Table (mroute) Check the mroute table on the LHR to ensure the (*, G) entries point toward the Anycast RP.

Use Case: ISP Multicast Content Delivery An ISP providing IPTV services needs to ensure that the stream remains uninterrupted even if a core RP node fails. By deploying Anycast RP on two geographically separate routers, the ISP achieves automatic redundancy. Receivers will join via the topologically closest RP, optimizing bandwidth while providing a robust backup solution.