2.1. Definitions

The following terms have special significance for PIM-SM:

Rendezvous Point (RP): An RP is a router configured to be used as the root of the non-source-specific distribution tree for a multicast group. Join messages from receivers for a group are sent towards the RP, and data from senders is sent to the RP so receivers can discover senders and start receiving traffic.

Designated Router (DR): A shared-media LAN may have multiple PIM-SM routers connected to it. A single one of these routers, the DR, will act on behalf of directly connected hosts with respect to the PIM-SM protocol. A DR is elected per interface using a simple election process.

MRIB: Multicast Routing Information Base. The multicast topology table, typically derived from the unicast routing table or routing protocols that carry multicast-specific topology information. Used to decide where to send Join/Prune messages and to provide routing metrics for Assert messages.

RPF Neighbor: RPF stands for "Reverse Path Forwarding". The RPF Neighbor of a router with respect to an address is the neighbor that the MRIB indicates should be used to forward packets to that address.

TIB: Tree Information Base. The collection of state at a PIM router created by receiving PIM Join/Prune messages, PIM Assert messages, and IGMP or MLD information from local hosts. Stores the state of all multicast distribution trees at that router.

MFIB: Multicast Forwarding Information Base. While the TIB holds all necessary state to forward multicast packets, routers build an efficient MFIB from the TIB state to perform actual forwarding.

Upstream: Towards the root of the tree. The root may be either the source or the RP.

Downstream: Away from the root of the tree.

GenID: Generation Identifier, used to detect reboots.