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A multicast routing protocol lets routers deliver IP packets for a multicast group only along network branches that lead to interested receivers. It builds and maintains the forwarding state—the distribution tree—that routers use to send traffic where it is needed and keep it off other branches.
What a multicast routing protocol does
With ordinary unicast, a sender addresses traffic to one destination. With multicast, a sender transmits to a group address, and multiple receivers can receive the same stream. The multicast routing protocol coordinates routers across the network so packets travel toward networks with interested receivers rather than being sent independently to each receiver or indiscriminately everywhere.
To do that, routers establish forwarding state and use it to decide where multicast packets should go. Depending on the protocol, branches without receivers may be excluded from the distribution tree or have traffic pruned after it has been sent there.
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How multicast routing differs from IGMP and MLD
IGMP and MLD are membership protocols, not router-to-router multicast routing protocols. They tell a nearby router that systems on its local network want to join or leave multicast groups. The router uses those reports as input when determining where multicast traffic should be forwarded through the wider network.
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| Protocol | Address family | Role |
|---|---|---|
| IGMP | IPv4 | Reports multicast group membership from systems to a neighboring router. |
| MLD | IPv6 | Provides the corresponding listener-discovery role for IPv6. |
| Multicast routing protocol | IPv4 or IPv6, depending on the protocol and deployment | Establishes network forwarding state and carries multicast traffic toward interested networks. |
IGMPv3 and MLDv2 support source filtering: a system can express interest in traffic from particular sources for a group, or exclude sources. This is useful when group membership alone is not specific enough.
How routers get multicast traffic to receivers
- A receiver signals local interest. A system joins a multicast group using IGMP on IPv4 or MLD on IPv6.
- The attached router learns what is wanted. It records the group interest on that network and, where supported, any source-filtering preferences.
- Routers establish forwarding state. The multicast routing protocol builds or updates the distribution tree to reach networks with interested receivers.
- Routers forward along the tree. Traffic follows that state, while branches without relevant receivers can be excluded or pruned according to the protocol.
The distinction is local signaling versus network-wide forwarding: IGMP or MLD tells a router what local systems want; multicast routing coordinates how routers deliver the traffic across the network.
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What PIM does
Protocol Independent Multicast (PIM) is a family of multicast routing protocols. PIM-Sparse Mode (PIM-SM) uses information from the unicast routing table to make reverse-path forwarding decisions, but it is not tied to a particular unicast routing protocol. “Protocol independent” describes that relationship; it does not mean PIM works without routing information.
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Sparse Mode is designed around the assumption that receivers are distributed such that not every network needs a stream. Routers signal interest and build delivery paths for requested traffic. PIM-SM supports both Any Source Multicast (ASM), where receivers subscribe to a group without specifying one source, and Source-Specific Multicast (SSM), where receivers identify the source and group they want.
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PIM-DM: flood and prune
Dense Mode starts by flooding multicast packets through the network, then uses prune messages to stop traffic from continuing toward routers that have no group-membership information. That flood-and-prune behavior differs from Sparse Mode’s explicit-interest approach. Neither mode is universally preferable; the fit depends on receiver distribution and network requirements.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What to consider when choosing an approach
- Receiver distribution: Sparse-mode designs use explicit interest signaling; dense-mode designs flood first and prune unwanted branches.
- Source model: Decide whether receivers need ASM or SSM. SSM depends on receivers knowing the source-and-group pair they want; source filtering can express selective source interest.
- Topology and resilience: RFC 4605 describes proxying as an option for simpler topologies. It says more complicated topologies, a need for more robust failover, or multiple administrative domains call for a multicast routing protocol.
- Address family: IPv4 deployments use IGMP for membership signaling; IPv6 uses MLD. Check the required protocol versions if source filtering matters.
- Platform support: Verify that the network devices and software support the multicast routing and membership features the design requires. Support varies by platform; a definition alone cannot establish what a particular deployment supports.
These factors matter more than choosing a protocol based on a general popularity claim. An IETF overview from 2008 called PIM-SM “by far, the most common multicast routing protocol,” but that is a historical characterization, not evidence of current adoption.
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Standards and specifications
- RFC 7761 specifies PIM-SM.
- RFC 3973 specifies PIM-DM.
- RFC 9776 specifies IGMPv3.
- RFC 4604 describes source-specific multicast and source filtering for IPv4 and IPv6.
- RFC 4605 specifies an IGMP/MLD-based multicast forwarding proxy and discusses when a routing protocol is more appropriate.
- RFC 5110 is the 2008 informational overview containing the historical PIM-SM prevalence description.
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