PIM Light

RFC section 4.3.1 describes the PIM neighbor discovery via Hello messages. It also describes that PIM Join/Prune are not accepted from a router unless a Hello message has been heard from that router. In some scenarios it is desired to communicate and build multicast states between two directly or non directly attach routers without establishing a PIM neighborship. There could be many reasons for this desired, but one example is the desired to signal multicast states upstream, between two or more PIM Domains via a network or medium that is not optimized for PIM or does not require PIM Neighbor establishment. An example is a BIER network connecting multiple PIM domains. In these BIER networks PIM Join/prune messages are tunneled via bier as per . A PIM Light Interface (PLI) does ONLY accept Join/Prune messages from an unknown PIM router and it accepts it without receiving a PIM Hello message form the router. Lack of Hello Messages on a PLI means there is no mechanism to learn about the neighboring PIM routers on each interface and there is no DR Priority options communicated between Routers either. As such the router doesn't create any General-Purpose state for neighboring PIM routers and it accepts and installs each Join message from upstream routers in its multicast routing table. Because of this a PLI needs to be created in very especial cases and the application that is using these PLIs should ensure there is no multicast duplication of packets. As an example, multiple upstream routers sending the same multicast stream to a single downstream router. As an example, in a BIER domain which is connecting 2 PIM networks a PLI can be used between the BIER edge routers. The PLI will be used for only multicast states communicated via by transmitting ONLY PIM Join/prunes over the BIER domain. In this case to ensure there is no multicast stream duplication the PIM routers attached on each side of the BIER domain might want to establish PIM Adjacency via to ensure DR selection on the edge of the BIER router, while PLI is used in the BIER Domain between BIER edge routers. When the Join or Prune message arrives from a PIM domain to the down stream BIER edge router, it can be send over the BIER tunnel to the upstream BIER edge router. These BIER edge routers have PLI established between them hence PIM Join/Prunes can be accepted on these routers to build their multicast state. After the multicast states are build the traffic will flow as per

Since a PLI doesn't require PIM Hello Messages and PIM neighbor adjacency is not checked for join/prune messages, there needs to be a mechanism to enable PLI on interfaces for security purpose, while on some other interfaces this may be enabled automatically. An example of the latter is the logical interface for a BIER sub-domain . If a system explicitly needs a PLI to be configured, then this system should not accepts the Join/Prune messages on interfaces that the PLI is not configured on, and it should drop these messages on a non PLI interface. If the system automatically enables PLI on some special interfaces, as an example interfaces facing a BIER domain, then it should accept Join/Prune messages on these interfaces only.