Protocol Independent Multicast (PIM)

 

Protocol Independent Multicast (PIM)

PIM is a multicast routing protocol that uses the forwarding table of an existing unicast routing protocol. It is called protocol independent because the underlying unicast protocol can be either distance-vector or link-state.

PIM has two modes:

Mode	When used	Tree approach
PIM-DM (Dense Mode)	Many routers have group members	Source-based tree
PIM-SM (Sparse Mode)	Few routers have group members	Group-shared tree

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1. PIM-DM — Dense Mode

PIM-DM is used when most routers are likely to have members of the multicast group.

It uses RPF + RPM:

1. RPF: Router checks whether the packet arrived through the correct interface—the interface leading back toward the source.
2. Forward: If valid, the router initially forwards the packet through its other interfaces.
3. Prune: Routers that have no members send prune messages upstream.
4. Unwanted branches are removed, gradually changing the initial broadcast into a multicast tree.

Simple idea:

First packet
     ↓
Broadcast to many routers
     ↓
Routers without members
send PRUNE
     ↓
Unwanted branches removed
     ↓
Multicast tree

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2. PIM-SM — Sparse Mode

PIM-SM is used when only a few routers have members of the multicast group.

It uses a group-shared tree. A special router called the Rendezvous Point (RP) acts as the center of the multicast group.

Basic operation:

1. A source sends its multicast packet to the RP using tunneling.
2. The RP forwards the packet toward the group members.
3. When a network gets a new member, its designated router sends a JOIN message toward the RP.
4. JOIN messages create branches of the multicast tree.
5. When a network no longer has members, its router sends a PRUNE message.
6. Unnecessary branches are removed.

             RP
            /  \
           /    \
         R1      R2
         |        |
       Group     Group
      Members   Members

JOIN  → creates a branch
PRUNE → removes a branch

Key Difference

PIM-DM: Send first, prune later — suitable for dense groups.
PIM-SM: Build the required tree using JOIN — suitable for sparse groups.

Key point: PIM is protocol independent because it does not depend on how the unicast forwarding table was created; it simply uses that table to determine the next router toward a destination/source.