How OSPF Builds Its View of the Network

Unlike distance-vector protocols (RIP, EIGRP) that only know "how far to get somewhere," OSPF is a link-state protocol, every router builds a complete map of the entire topology. This map is called the LSDB (Link State Database). Once every router has an identical LSDB, they each independently run Dijkstra's shortest-path-first (SPF) algorithm to calculate the best routes.

This is why OSPF converges faster and avoids routing loops: every router has complete information, not just a rumor about the best next hop.

LSA Types: The Building Blocks of the LSDB

The LSDB is populated by LSAs. There are 7 relevant LSA types, knowing which does what is critical for troubleshooting:

Type 1, Router LSA

Generated by every OSPF router. Describes the router's directly connected links and their states. Flooded only within the originating area.

Type 2, Network LSA

Generated by the DR (Designated Router) on multi-access networks (Ethernet). Represents the network segment and lists all attached routers. Also area-local.

Type 3, Summary LSA

Generated by ABRs (Area Border Routers). Carries route information between areas. This is how Area 1 learns about routes in Area 2, the ABR translates Type 1/2 LSAs into Type 3s and floods them into connected areas.

Type 4, ASBR Summary LSA

Also generated by ABRs. Tells routers in other areas how to reach an ASBR (Autonomous System Boundary Router), a router redistributing external routes into OSPF.

Type 5, AS External LSA

Generated by ASBRs. Carries external route information (routes redistributed from BGP, static routes, etc.) and flooded to the entire OSPF domain (except stub areas).

Type 7, NSSA External LSA

Like Type 5, but used within NSSA (Not-So-Stubby Areas). The ABR converts Type 7s to Type 5s when forwarding them to the backbone.

Area Design: Why It Matters

In a small network, one area (Area 0) is fine. But as networks grow, a flat OSPF design becomes a problem. Every topology change triggers an SPF recalculation on every router. With hundreds of routers and frequent changes, this is CPU-intensive and increases convergence time.

The solution: hierarchical area design. All areas connect to Area 0 (the backbone). ABRs sit at the boundary.

Area Types and Their Purpose

  • Backbone (Area 0): All inter-area traffic must transit Area 0. ABRs have one interface in Area 0.
  • Regular Area: Accepts all LSA types. Standard configuration.
  • Stub Area: Blocks Type 5 (external) LSAs. ABR injects a default route instead. Reduces LSDB size for areas with no need for external routes.
  • Totally Stubby Area (Cisco): Blocks Type 3, 4, and 5. Only a default route enters. Smallest possible LSDB. Best for remote branch areas with a single exit point.
  • NSSA: Allows redistribution of external routes within the area (via Type 7 LSAs) while still blocking Type 5s. Used when an area needs its own ASBR.

DR and BDR Elections

On multi-access networks (Ethernet segments with 3+ OSPF routers), OSPF elects a Designated Router (DR) and Backup DR (BDR). Without this, every router would form a full mesh of adjacencies, n(n-1)/2 sessions.

Instead, all routers form adjacencies only with the DR and BDR. The DR is responsible for generating the Type 2 LSA and flooding updates on behalf of the segment.

Election process:

  1. Highest OSPF priority (0-255, default 1) wins. Priority 0 = never become DR.
  2. Tie-break: highest Router ID (highest loopback IP, or highest interface IP).
  3. Election is non-preemptive once complete. A new, higher-priority router won't unseat an existing DR without a network event.
! Force a router to always become DR
interface GigabitEthernet0/0
 ip ospf priority 255

! Prevent a router from ever becoming DR
interface GigabitEthernet0/0
 ip ospf priority 0

The SPF Algorithm: Dijkstra Under the Hood

Once all LSAs are received and the LSDB is complete, OSPF runs Dijkstra's algorithm. The process:

  1. Place the local router in the SPF tree as root with cost 0
  2. Examine all links from the current node; calculate cumulative cost to each neighbor
  3. Add the lowest-cost unvisited neighbor to the tree
  4. Repeat from step 2 until all nodes are in the tree

The result is a shortest-path tree rooted at your router. OSPF cost is calculated as: reference bandwidth / interface bandwidth. Default reference: 100 Mbps.

! This means:
! FastEthernet (100Mbps): cost = 100/100 = 1
! GigabitEthernet (1Gbps): cost = 100/1000 = 0.1 → rounded up to 1
! (same as FastEthernet! Increase reference bandwidth)

router ospf 1
 auto-cost reference-bandwidth 10000   ! 10 Gbps reference
 ! Now GigabitEthernet cost = 10000/1000 = 10
 ! 10GigE cost = 10000/10000 = 1

OSPF Timers and Convergence Tuning

OSPF uses two critical timers for detecting link failures:

  • Hello interval: How often HELLO packets are sent (default 10s on broadcast, 30s on NBMA)
  • Dead interval: How long without a HELLO before the neighbor is declared dead (default 4× hello = 40s)

For fast convergence, you can reduce these, but both sides must match:

interface GigabitEthernet0/0
 ip ospf hello-interval 1
 ip ospf dead-interval 3

For sub-second convergence, use BFD (Bidirectional Forwarding Detection) alongside OSPF. BFD detects link failures in milliseconds and notifies OSPF to trigger reconvergence immediately.

Troubleshooting Adjacency Issues

OSPF neighbors must agree on: hello/dead timers, area ID, authentication, stub area flag, and MTU (with ip ospf mtu-ignore if needed). The most common stuck state is 2WAY, this is normal for non-DR/BDR routers on broadcast networks. The full adjacency state you want is FULL.

! Key diagnostic commands
show ip ospf neighbor          ! Check neighbor states
show ip ospf database          ! View the LSDB
show ip ospf interface brief   ! Verify areas, costs, DR/BDR
debug ip ospf adj              ! Watch adjacency formation (use carefully!)

Key Takeaways

  • OSPF builds a complete topology map (LSDB) from LSAs; each router runs SPF independently
  • Know your LSA types 1-5 and 7, they're essential for troubleshooting inter-area routing
  • Use stub/totally stubby areas to reduce LSDB size in areas with single exit points
  • Always set reference bandwidth to match your fastest links
  • Use BFD for sub-second failure detection