A unicast routing protocol is a networking protocol that helps routers determine paths for packets addressed to a single destination. The protocol distributes or calculates reachability information. Each router then uses that information in its forwarding table to send each packet toward the next hop it has selected. The term names a category of protocols, not one specific protocol.
What “unicast” means in this definition
Unicast describes one-to-one delivery: a packet carries a single destination IP address, and the network tries to deliver it to that one host or interface. In contrast, multicast delivers a packet to a group of receivers. The word “unicast” therefore tells you who the traffic is for, not how the routing protocol works internally.
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What a routing protocol does
A routing protocol is a set of rules that routers follow to exchange or compute information about which destinations are reachable and by which paths. This is control-plane work. It is separate from forwarding, which is the act of moving one packet. A router forwards each packet by looking up the destination in its forwarding table and sending the packet to the selected next hop. The routing protocol is what fills and updates that table.
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OSPF as a worked example
OSPF (Open Shortest Path First) is a convenient concrete example, but it is one member of the category, not the definition itself. RFC 2328, the OSPF Version 2 specification published by the IETF in April 1998, states: “OSPF routes IP packets based solely on the destination IP address found in the IP packet header.” That sentence describes destination-based unicast forwarding in practice.
OSPF is a dynamic, link-state Interior Gateway Protocol. It works inside a single autonomous system, which is a network under one administrative domain. Its routers follow a predictable cycle:
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- Discover and describe local links. Each router records the state of its own attached links.
- Distribute link-state information. Routers flood this information to other routers in the same area.
- Build a topology database. Every router holds the same picture of the network’s links and routers.
- Calculate shortest paths. Each router runs a shortest-path calculation over that database to select routes.
- Recalculate after changes. When a link or router changes state, routers update the database and recompute routes.
The link-state method is what distinguishes OSPF from protocols that exchange only distance or next-hop summaries. A reader who understands this cycle has a clear model of one unicast routing approach, even though many other approaches exist.
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Multicast routing is a different delivery pattern, and the two should not be treated as synonyms. The table below summarises the main differences as described in the IETF’s multicast architecture overview (RFC 5110, January 2008).
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| Aspect | Unicast routing | Multicast routing |
|---|---|---|
| Intended receivers | One destination address | Members of a destination group |
| Information the routing decision uses | Reachability to the destination | Source and group destination together |
| Typical example in this article | OSPF forwarding by destination IP address | Not covered here beyond this comparison |
Where the definition stops
Avoid assuming that every unicast routing protocol behaves like OSPF. Protocols in this category can differ in several ways:
- Scope: interior protocols run within one autonomous system, while inter-domain protocols such as BGP exchange routes between autonomous systems.
- Route-selection method: some protocols compute shortest paths from a full topology, others rely on distance or path information from neighbours.
- Metrics and policy: the inputs used to choose a path can include cost, prefix, or administrative policy.
- Convergence behaviour: the speed and manner in which routers react to topology changes vary.
The sources behind this article establish OSPF’s interior scope and link-state approach. They do not provide a full, current side-by-side comparison with BGP or other protocols, so treat any such comparison as a separate topic. For an introductory understanding, the shared purpose and OSPF as a labelled example are enough.
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Sources
- RFC 2328, OSPF Version 2, RFC Editor / IETF, April 1998. Primary protocol specification.
- RFC 5110, Overview of the Internet Multicast Routing Architecture, RFC Editor / IETF, January 2008. Informational architecture overview.
- RFC 2178, OSPF Version 2, RFC Editor / IETF, July 1997. Earlier specification that defines a router as “A level three Internet Protocol packet switch” and describes an Interior Gateway Protocol as the protocol spoken by routers in one autonomous system. It has since been superseded by RFC 2328, so cite it for historical definitions only.
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