OSPF Concepts for JNCIS-SP

Wed, 15 Apr 2026 00:00:00 +0000

OSPF (Open Shortest Path First)

OSPF is a link-state interior gateway protocol (IGP). Each router floods Link-State Advertisements (LSAs) describing its interfaces and neighbors. Every router builds an identical Link-State Database (LSDB) and runs the Dijkstra SPF algorithm to compute the shortest path tree. OSPF runs directly over IP (protocol 89) and uses multicast for efficiency.

Default route preferences in Junos:

OSPF Internal routes: 10
OSPF AS External routes: 150

Terms

LSDB (Link-State Database) - The topological database. Within a single area, all routers must have an identical LSDB.
SPF (Shortest Path First) - The Dijkstra algorithm each router runs against the LSDB to compute best paths.
Router ID (RID) - A 32-bit identifier unique to each OSPF router. Junos selects the RID in this order: explicitly configured → highest active loopback IP → highest physical interface IP. Best practice is to configure it explicitly.
ABR (Area Border Router) - A router with interfaces in multiple OSPF areas. Generates Type 3 (Summary) LSAs between areas.
ASBR (AS Boundary Router) - A router that redistributes routes from outside OSPF into the OSPF domain. Generates Type 5 LSAs.
Backbone Router - Any router with at least one interface in Area 0.
Internal Router - All interfaces are in the same single area.

set routing-options router-id 4.4.4.4

OSPF Packet Types

OSPF has five packet types. All run directly over IP (protocol 89) — no TCP/UDP — so reliability is handled by LSAcks.

Ospf on TTL Expired

OSPF Concepts for JNCIS-SP

OSPF (Open Shortest Path First)

Terms

OSPF Packet Types