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PIM
A family of routing protocols used to efficiently distribute multicast traffic (one-to-many or many-to-many data streams) across IP networks.
PIM operates in several distinct modes depending on how receivers are distributed across the network and how data paths are built.
| PIM Mode | Traffic Distribution Model | Key Characteristics & Mechanics |
|---|---|---|
| PIM Sparse Mode (PIM-SM) | Pull Model (Explicit Join) | Traffic is sent only to routers that actively request it. It relies on a central node called a Rendezvous Point (RP) to connect sources and receivers. Ideal for sparse receiver networks. |
| PIM Source-Specific Multicast (PIM-SSM) | Direct Pull (No RP Needed) | Receivers explicitly request traffic from a specific source (S,G). It bypasses the need for an RP entirely, making it highly secure and efficient for modern deployments. |
| PIM Dense Mode (PIM-DM) | Push Model (Flood & Prune) | Traffic is flooded to every corner of the network. Routers with no active receivers must explicitly send "prune" messages to stop the flow. Rarely used today due to high bandwidth waste. |
PIM constructs trees to deliver data:
- Shared Tree (*,G): Used in PIM-SM to route traffic from any source through the Rendezvous Point to the receivers.
- Source Tree (S,G): A direct shortest-path tree (SPT) from the specific source to the receivers, bypassing the RP to reduce latency.
Reverse Path Forwarding (RPF) is a core security and loop-prevention check used by routers to safely forward multicast traffic.
[ Multicast Packet Arrives ]
│
▼
What is the Source IP?
│
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Check the Unicast Routing Table:
"Which interface would I use to SEND a unicast packet BACK to this Source IP?"
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┌────────┴────────┐
▼ ▼
[ Match! ] [ Mismatch! ]
Packets arrived Packet arrived on a
on that exact different interface.
interface. │
│ ▼
▼ [ RPF FAILURE ]
[ RPF SUCCESS ] Drop the packet
Forward packet immediately.
to receivers.
The logic behind requiring an exact interface match comes down to a fundamental law of IP networks: unicast routing tables only calculate the single best path back to a destination.
If a multicast packet arrives on any interface other than that single best path, it means the packet took a detour, a backup route, or is caught in a routing loop.
The Multicast Source Discovery Protocol (MSDP) is an inter-domain routing protocol defined in RFC 3618 that connects multiple Protocol Independent Multicast Sparse-Mode (PIM-SM) domains.