Configuring Different PIM Modes
SUMMARY Configuring different PIM modes in Junos OS.
PIM operates in four modes. The PIM mode you choose determines the PIM configuration properties, therefore you first must decide which mode PIM operates in your network. Each mode has distinct operating advantages in different network environments.
Sparse Mode
In sparse mode, routing devices must join and leave multicast groups explicitly. Upstream routing devices do not forward multicast traffic to a downstream routing device unless the downstream routing device has sent an explicit request (by means of a join message) to the rendezvous point (RP) routing device to receive this traffic. The RP serves as the root of the shared multicast delivery tree and is responsible for forwarding multicast data from different sources to the receivers.
Sparse mode is well suited to the Internet, where frequent interdomain join messages and prune messages are common. Refer to Examples: Configuring PIM Sparse Mode to learn how to configure PIM in sparse mode.
Bidirectional Mode
Bidirectional PIM is similar to sparse mode, and is especially suited to applications that must scale to support a large number of dispersed sources and receivers. In bidirectional PIM, routing devices build shared bidirectional trees and do not switch to a source-based tree. Bidirectional PIM scales well because it needs no source-specific (S,G) state. Instead, it builds only group-specific (*,G) state.
Refer to Example: Configuring Bidirectional PIM to learn how to configure PIM in bidirectional mode.
Dense Mode
Unlike sparse mode and bidirectional mode, in which data is forwarded only to routing devices sending an explicit PIM join request, dense mode implements a flood-and-prune mechanism, similar to the Distance Vector Multicast Routing Protocol (DVMRP). In dense mode, a routing device receives the multicast data on the incoming interface, then forwards the traffic to the outgoing interface list. Flooding occurs periodically and is used to refresh state information, such as the source IP address and multicast group pair. If the routing device has no interested receivers for the data, and the outgoing interface list becomes empty, the routing device sends a PIM prune message upstream. Dense mode works best in networks where few or no prunes occur. In such instances, dense mode is actually more efficient than sparse mode.
Refer to Configuring PIM Dense Mode to learn how to configure PIM in dense mode.
Sparse-Dense Mode
Sparse-dense mode, as the name implies, allows the interface to operate on a per-group basis in either sparse or dense mode. A group specified as “dense” is not mapped to an RP. Instead, data packets destined for that group are forwarded by means of PIM dense mode rules. A group specified as “sparse” is mapped to an RP, and data packets are forwarded by means of PIM sparse-mode rules. Sparse-dense mode is useful in networks implementing auto-RP for PIM sparse mode.
Refer to Configuring PIM Sparse-Dense Mode to learn how to configure PIM in sparse-dense mode.