PIM Overview

The predominant multicast routing protocol in use on the Internet today is Protocol Independent Multicast, or PIM. The type of PIM used on the Internet is PIM sparse mode. PIM sparse mode is so accepted that when the simple term “PIM” is used in an Internet context, some form of sparse mode operation is assumed.

PIM emerged as an algorithm to overcome the limitations of dense-mode protocols such as the Distance Vector Multicast Routing Protocol (DVMRP), which was efficient for dense clusters of multicast receivers, but did not scale well for the larger, sparser, groups encountered on the Internet. The Core Based Trees (CBT) Protocol was intended to support sparse mode as well, but CBT, with its all-powerful core approach, made placement of the core critical, and large conference-type applications (many-to-many) resulted in bottlenecks in the core. PIM was designed to avoid the dense-mode scaling issues of DVMRP and the potential performance issues of CBT at the same time.

Starting in Junos OS Release 15.2, only PIM version 2 is supported. In the CLI, the command for specifying a version (1 or 2) is removed.

PIMv1 and PIMv2 can coexist on the same routing device and even on the same interface. The main difference between PIMv1 and PIMv2 is the packet format. PIMv1 messages use Internet Group Management Protocol (IGMP) packets, whereas PIMv2 has its own IP protocol number (103) and packet structure. All routing devices connecting to an IP subnet such as a LAN must use the same PIM version. Some PIM implementations can recognize PIMv1 packets and automatically switch the routing device interface to PIMv1. Because the difference between PIMv1 and PIMv2 involves the message format, but not the meaning of the message or how the routing device processes the PIM message, a routing device can easily mix PIMv1 and PIMv2 interfaces.

PIM is used for efficient routing to multicast groups that might span wide-area and interdomain internetworks. It is called “protocol independent” because it does not depend on a particular unicast routing protocol. Junos OS supports bidirectional mode, sparse mode, dense mode, and sparse-dense mode.

PIM operates in several modes: bidirectional mode, sparse mode, dense mode, and sparse-dense mode. In sparse-dense mode, some multicast groups are configured as dense mode (flood-and-prune, [S,G] state) and others are configured as sparse mode (explicit join to rendezvous point [RP], [*,G] state).

PIM drafts also establish a mode known as PIM source-specific mode, or PIM SSM. In PIM SSM there is only one specific source for the content of a multicast group within a given domain.

Because the PIM mode you choose determines the PIM configuration properties, you first must decide whether PIM operates in bidirectional, sparse, dense, or sparse-dense mode in your network. Each mode has distinct operating advantages in different network environments.

• 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.

  Starting in Junos OS Release 19.2R1, on SRX300, SRX320, SRX340, SRX345, SRX550, SRX1500, and vSRX Virtual Firewall 2.0 and vSRX Virtual Firewall 3.0 (with 2 vCPUs) Series devices, Protocol Independent Multicast (PIM) using point-to-multipoint (P2MP) mode supports AutoVPN and Auto Discovery VPN in which a new p2mp interface type is introduced for PIM. The p2mp interface tracks all PIM joins per neighbor to ensure multicast forwarding or replication only happens to those neighbors that are in joined state. In addition, the PIM using point-to-multipoint mode supports chassis cluster mode.

  Note:

  On all the EX series switches (except EX4300 and EX9200), QFX5100 switches, and OCX series switches, the rate limit is set to 1pps per S,G multicast group to avoid overwhelming the rendezvous point (RP), First hop router (FHR) with PIM-sparse mode (PIM-SM) register messages and cause CPU hogs. This rate limit helps in improving scaling and convergence times by avoiding duplicate packets being trapped, and tunneled to RP in software. (Platform support depends on the Junos OS release in your installation.)

• 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.

• 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.

• 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.