PIM Terminlogy

Reverse Path Forwarding (RPF) Interface

The reverse path forwarding interface is the interface with the lowest cost path to the IP address of the root of the shortest path tree (the source of the multicast stream).

The lowest cost is based on the factors of administrative distance and metric. If there are multiple interfaces with the same cost, a tiebreaker is carried out with the interface of the highest IP address elected as the preferred path.

RPF Neighbour

The RPF neighbour is the PIM neighbour on the RPF interface.


Upstream works towards the source of the tree. This can be towards the source of the multicast traffic in a shortest path tree or towards the router rendezvous point in a shared tree.

Upstream Interface

The upstream interface is the interface towards the source of a tree. It cna be known as the RPF interface or an incoming interface.


Downstream is away from the source of the tree towards the receiving host.

Downstream Interface

A downstream interface is an interface that is used to forward multicast traffic towards the receiving hosts.

Incoming Interface (IIF)

An incoming interface accepts multicast traffic coming from a source of multicast traffic. It is the same as an RPF interface.

Outgoing Interface (OIF)

An outgoing interface is used to forward multicast traffic down the tree towards the receiving hosts.

Outgoing Interface List (OIL)

An outgoing interface list is a collection of outgoing interfaces that are forwarding multicast traffic to the same group.

Last Hop Router (LHR)

The last hop router is the router that is directly attached to receiving hosts. It is also known as the leaf router. It sends PIM joins upstream towards the rendezvous or the source.

First Hop Router (FHR)

The first hop router is directly attached to the multicast source. It is also known as the root router. It sends register messages towards the rendezvous point.

Multicast Routing Information Base (MRIB)

The multicast routing information base is a topology table, also known as the multicast route table, or mroute. The multicast routing information base contains the source S, group G, incoming interfaces, outgoing interfaces, and reverse path forwarding for each multicast router.

Multicast Forwarding Information Base (MFIB)

The multicast forwarding information base contains multicast forwarding information to make forwarding faster through hardware.

Multicast State

The multicast forwarding state is used by a router to forward multicast traffic. It is composed of entries found in the mroute table (S, G, IIF, OIF)




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