OSPF Link Types

OSPF (Open Shortest Path First) is a link-state interior gateway protocol. In its topology database, OSPF does not model just point-to-point connections — every adjacency is classified as one of four link types.

Basis of classification

The classification is based on (a) how the two endpoints (routers) are connected — i.e., the nature of the connection between the routers, and (b) whether each endpoint has a neighbour on that link / whether both ends belong to the same routing domain (area). In practice OSPF decides the type from the configured interface type and whether the far end is another router, a network segment, or an AS boundary.

The four link types

Link type	Description
Point-to-point link	Connects exactly two routers directly (e.g., a serial line / leased line). No other host or router is on the link, so no addressing/broadcast issue arises. Both routers become adjacent and exchange LSAs directly.
Transit link (broadcast network)	A network (e.g., Ethernet, Token Ring) that can support multiple (three or more) routers and broadcast/multicast. A Designated Router (DR) and Backup DR (BDR) are elected to limit the number of adjacencies (full mesh avoided) and to flood LSAs to all attached routers.
Stub link	A network that connects to exactly one router (a "dead end"). Hosts on such a link reach the rest of the internetwork only through that single router. OSPF advertises it as a stub network (LS type 3) — it is not used to reach other routers.
Virtual link	A logical, configured link through a non-backbone area to connect an area that is physically disconnected from area 0 (backbone). It acts like a point-to-point link, but is really a tunnel through one or more intermediate routers; must run within a single area.

Why the classification matters

• Point-to-point vs transit determines whether OSPF forms adjacencies with every neighbour or only with the DR/BDR — this controls LSA flooding overhead.
• Stub links are simply advertised as reachable networks, avoiding unnecessary router-to-router routing.
• Virtual links keep the rule "all areas must be physically connected to area 0" satisfied even in poorly planned topologies.

In short, OSPF classifies links by the number/kind of endpoints attached to the segment and the connectivity role of the link in the OSPF hierarchy.