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A unicast routing protocol is a networking protocol that helps routers work out which paths to use for packets addressed to a single destination. Routers do not carry packets on their own in a vacuum. They exchange or calculate reachability information, store the result in a forwarding table, and then send each packet toward the next hop that table selects. OSPF (Open Shortest Path First) is a widely documented example of this category, but it illustrates the definition rather than defining it.
Breaking down the term
The phrase has two parts, and each one narrows the meaning.
“Unicast”: one destination
Unicast describes one-to-one delivery. A packet carries a destination IP address that identifies a single target, and the network’s job is to move that packet toward it. This is the normal mode for most web requests, email transfers and file downloads. In contrast, multicast delivers a packet to a group of receivers.
“Routing protocol”: how routers learn paths
A routing protocol is a set of rules that routers follow to exchange information about which destinations they can reach and at what cost. RFC 2178 defines a router as “A level three Internet Protocol packet switch.” Routers need some way to learn about destinations beyond their directly connected networks, and routing protocols provide that information. Static routes configured by hand are not protocols in this sense because nothing is exchanged between routers.
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Routing protocols and packet forwarding are different jobs
A common source of confusion is treating the routing protocol as if it moves the packet. It does not. The work splits into two parts:
- Control plane (routing): the routing protocol gathers or computes reachability information and builds the routes the router will use.
- Forwarding plane: for each arriving packet, the router looks up the destination address in its forwarding table and sends the packet to the selected next hop.
The protocol decides what the forwarding table should contain. The forwarding decision is then made packet by packet. Keeping these two roles separate makes it easier to troubleshoot a problem, because a route can be missing from the table (a control-plane issue) even when forwarding itself works correctly.
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OSPF as a concrete example
OSPF is an interior gateway protocol (IGP), meaning it runs among routers inside one autonomous system, which is a network under a single administrative domain. RFC 2178 describes an IGP as the protocol spoken by routers belonging to an autonomous system. RFC 2328, the OSPF Version 2 specification published in April 1998, describes OSPF as a dynamic, link-state routing protocol.
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How OSPF’s link-state method works
- Each router learns about its own directly attached links and their state.
- Routers distribute this link-state information to the other routers in the area.
- Every router assembles the same topology database from these advertisements.
- Each router runs a shortest-path calculation over that database to produce its routes.
- When the topology changes, routers distribute the change and recalculate their routes.
RFC 2328 states the destination-based behaviour directly: “OSPF routes IP packets based solely on the destination IP address found in the IP packet header.” That sentence is what makes OSPF a unicast routing protocol in practice: each packet is routed by its single destination address.
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Unicast compared with multicast
| Aspect | Unicast routing | Multicast routing |
|---|---|---|
| Intended receivers | One destination address | Members of a destination group |
| Route state considered | Destination address and path to it | Source and group destination (per RFC 5110’s architecture overview) |
| Example protocol cited in this article | OSPF (RFC 2328) | Not covered by the sources used here; see RFC 5110 for the multicast architecture |
The two should not be used as synonyms. A protocol that builds routes toward group members is doing a different job from one that builds routes toward a single address, even though both are routing in the general sense.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What the definition does not cover
The category is broad, and individual protocols differ in several ways:
- Scope: interior protocols such as OSPF operate within an autonomous system, while inter-domain protocols operate between autonomous systems.
- Route-selection inputs: some protocols choose routes mainly by metric, others also weigh policy.
- Convergence behaviour: the way changes are distributed and routes recalculated varies between protocols.
A full side-by-side comparison of OSPF with BGP or other protocols is outside what the sources cited here establish, so treat any such comparison as a separate topic. For a definition, the essential points are the shared purpose (choosing paths for traffic to individual destinations) and OSPF as one example of how that purpose is implemented.
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Sources
- RFC 2328, OSPF Version 2, RFC Editor / IETF, April 1998 (primary protocol specification).
- RFC 2178, OSPF Version 2, RFC Editor / IETF, July 1997 (historical specification with router and IGP definitions).
- RFC 5110, Overview of the Internet Multicast Routing Architecture, RFC Editor / IETF, January 2008 (informational overview of multicast routing).
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