Protocol Independent Multicast-Sparse Mode

Note

PIM is supported in Global Routing Table (GRT) only.

PIM-SM, as defined in RFC2362, supports multicast groups spread out across large areas of a company or the Internet. PIM-SM sends multicast traffic only to routers that specifically join a multicast group. This technique reduces traffic flow over WAN links and overhead costs for processing unwanted multicast packets.

Dense-mode protocols use a flood-and-prune technique, which is efficient with densely-populated receivers. However, for sparsely populated networks, PIM-SM is more efficient because it sends multicast traffic only to those routers that belong to a specific multicast group and that choose to receive the traffic.

PIM-SM is independent of a specific unicast routing protocol, but it does require the presence of a unicast routing protocol, such as Routing Information Protocol (RIP) or Open Shortest Path First (OSPF). PIM-SM uses the information from the unicast routing table to create and maintain multicast trees that enable PIM-enabled routers to communicate.

Typically, a PIM-SM network consists of several multipoint data streams, each targeted to a small number of LANs in the internetwork. For example, customers whose networks consist of multiple hosts on different LANs can use PIM-SM to simultaneously access a video data stream, such as video conferencing, on a different subnet.

Important

In some cases, PIM stream initialization can take several seconds.

Hosts

A host is a source, a receiver, or both:

• A source, also known as a sender, sends multicast data to a multicast group.

• A receiver receives multicast data from one or several sources that sends data to a multicast group.

PIM-SM domain

PIM-SM operates in a domain of contiguous routers on which PIM-SM is enabled.

Each PIM-SM domain requires the following routers:

• designated router (DR)

• rendezvous point (RP) router

• bootstrap router (BSR)

Although a PIM-SM domain can use only one active RP router and one active BSR, you can configure additional routers as a candidate RP (C-RP) router and as a candidate BSR (C-BSR). Candidate routers provide backup protection in case the primary RP router or BSR fails.

As a redundancy option, you can configure several RPs for the same group in a PIM domain. As a load sharing option, you can have several RPs in a PIM-SM domain map to different groups. The switch devices use the hash function defined in the PIM-SM standard to elect the active RP.

Designated router

The designated router (DR), the router with the highest IP address on a LAN, performs the following tasks:

• sends register messages to the RP router on behalf of directly connected sources

• sends join and prune messages to the RP router on behalf of directly connected receivers

• maintains information about the status of the active RP router for local sources in each multicast group

Important

The DR is not a required configuration. Switches act automatically as the DR for directly attached sources and receivers.