A multicast routing protocol lets routers deliver IP packets addressed to a multicast group only across network branches that need them. It builds and maintains the forwarding state—the distribution tree—that carries traffic from sources toward networks with interested receivers.
What a multicast routing protocol does
With ordinary unicast, a packet is sent to one destination. With multicast, a source sends traffic to a group address, and routers use multicast forwarding state to replicate packets along the paths needed to reach interested networks. Branches without interested receivers can be excluded or pruned, depending on the protocol. The router-to-router mechanism that creates and maintains this state is multicast routing. IETF RFC 7761 specifies Protocol Independent Multicast (PIM) Sparse Mode, one example of such a protocol.
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How membership reports and routing work together
Multicast routing relies on a separate local signal: receivers tell a nearby router which groups they want to join. The router uses those reports as input when deciding where multicast traffic should go through the wider network.
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1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errors- A receiver signals interest in a multicast group on its local network.
- The attached router learns which groups—and, where source filtering is supported, which sources—have interested receivers.
- The multicast routing protocol establishes forwarding state between routers to carry traffic toward those networks.
- Routers forward packets along the resulting tree, excluding or pruning branches that do not need them.
The distinction is between local receiver membership and network-wide forwarding. IGMP serves the membership role for IPv4; MLD serves the corresponding role for IPv6. Neither replaces a multicast routing protocol.
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What PIM does
Protocol Independent Multicast is a family of multicast routing protocols. “Protocol independent” means PIM can use information from the network’s unicast routing system without depending on one particular unicast routing protocol. In PIM Sparse Mode, unicast routing information is used to determine reverse-path information for multicast forwarding. RFC 7761 defines PIM-SM.
Sparse mode: PIM-SM
Sparse-mode designs assume receivers are distributed so that flooding traffic everywhere is not the right starting point. Routers build delivery paths based on interest. PIM-SM supports both Any Source Multicast (ASM) and Source-Specific Multicast (SSM); the source model affects how receivers and routers identify the traffic to be delivered. The protocol details are specified in RFC 7761.
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Dense mode: PIM-DM
PIM Dense Mode begins by flooding multicast traffic through the network, then uses prune messages to stop traffic from continuing toward routers without group-membership information. This flood-and-prune behavior differs from sparse mode’s interest-driven approach. See RFC 3973 for the PIM-DM specification.
ASM, SSM and source filtering
ASM and SSM describe different source models for multicast. In ASM, receivers join a group without specifying one source. In SSM, a receiver identifies the source and group it wants. Source filtering lets a system express which sources it wants to receive—or which sources to exclude—for a group.
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For IPv4, IGMPv3 supports source filtering; for IPv6, MLDv2 provides the corresponding capability. The relationship between these membership mechanisms and source-specific multicast is described in RFC 4604. Membership signaling tells the local router what receivers want; multicast routing still handles forwarding through the network.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.When multicast routing is needed instead of proxying
A simpler topology may be served by multicast proxying, which relays membership information between network segments. The IETF’s RFC 4605 says a multicast routing protocol is appropriate when the topology is more complicated, stronger failover is needed, or multiple administrative domains are involved. The right choice depends on the actual network; the definition alone does not establish a universally best protocol or mode.
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Quick distinction
- IGMP: IPv4 receivers report multicast group membership to a neighboring router.
- MLD: IPv6 receivers report multicast group membership to a neighboring router.
- Multicast routing protocol: routers establish and use forwarding state to carry multicast traffic across the network.
- PIM: a family of multicast routing protocols, including sparse-mode and dense-mode approaches.
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