Basic PIM Network Components

PIM dense mode requires only a multicast source and series of multicast-enabled routers running PIM dense mode to allow receivers to obtain multicast content. Dense mode makes sure that everything gets everywhere by periodically flooding the network with multicast traffic, and relies on prune messages to make sure that subnets where all receivers are uninterested in that particular multicast group stop receiving packets.

PIM sparse mode is more complicated, and requires the establishment of special routers called rendezvous points (RPs) in the network core. These routers are where upstream join messages from interested receivers meet downstream traffic from the source of the multicast group content. A network can have many RPs, but PIM sparse mode allows only one RP to be active for any multicast group.

If there is only one RP in a routing domain, the RP and adjacent links might become congested and form a single point of failure for all multicast traffic. So multiple RPs are the rule, but the issue then becomes how other multicast routers find the RP that is the source of the multicast group the receiver is trying to join. This RP-to-group mapping is controlled by a special bootstrap router (BSR) running the PIM BSR mechanism. There can be more than one bootstrap router as well, also for single-point-of-failure reasons.

The bootstrap router does not have to be an RP itself, although this is a common implementation. The bootstrap router's main function is to manage the collection of RPs and allow interested receivers to find the source of their group's multicast traffic.

PIM SSM can be seen as a subset of a special case of PIM sparse mode and requires no specialized equipment other than that used for PIM sparse mode (and IGMP version 3).