2.1. Definitions

The following terms have special significance for PIM-SM:

Rendezvous Point (RP)

An RP is a router that has been 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 that receivers can discover who the senders are and start to receive traffic destined for the group.

Designated Router (DR)

A shared-media LAN like Ethernet 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 single DR is elected per interface (LAN or otherwise) using a simple election process.

MRIB

Multicast Routing Information Base. This is the multicast topology table, which is typically derived from the unicast routing table, or routing protocols such as Multiprotocol BGP (MBGP) that carry multicast-specific topology information. In PIM-SM, the MRIB is used to decide where to send Join/Prune messages. A secondary function of the MRIB is to provide routing metrics for destination addresses; these metrics are used when sending and processing 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. In the case of a PIM-SM multicast group, the RPF neighbor is the router that a Join message for that group would be directed to, in the absence of modifying Assert state.

TIB

Tree Information Base. This is the collection of state at a PIM router that has been created by receiving PIM Join/Prune messages, PIM Assert messages, and Internet Group Management Protocol (IGMP) or Multicast Listener Discovery (MLD) information from local hosts. It essentially stores the state of all multicast distribution trees at that router.

MFIB

Multicast Forwarding Information Base. The TIB holds all the state that is necessary to forward multicast packets at a router. However, although this specification defines forwarding in terms of the TIB, to actually forward packets using the TIB is very inefficient. Instead, a real router implementation will normally build an efficient MFIB from the TIB state to perform forwarding. How this is done is implementation-specific and is not discussed in this document.

Upstream

Towards the root of the tree. The root of the tree may be either the source or the RP, depending on the context.

Downstream

Away from the root of the tree.

GenID

Generation Identifier, used to detect reboots.