OSPF — Open Shortest Path First

Overview

OSPF is a link-state Interior Gateway Protocol (IGP) that uses Dijkstra’s Shortest Path First (SPF) algorithm to compute routes. It’s the most widely deployed IGP in enterprise networks.

• Protocol number: IP protocol 89
• RFC: RFC 2328 (OSPFv2), RFC 5340 (OSPFv3 for IPv6)
• Metric: Cost (inversely proportional to bandwidth)
• Algorithm: Dijkstra’s SPF
• AD: 110
• Multicast address: 224.0.0.5 (all OSPF routers), 224.0.0.6 (DR/BDR)

Why OSPF Matters

• Fast convergence (seconds, not minutes like RIP)
• Hierarchical design with areas for scalability
• Supports VLSM and CIDR
• No hop count limit (unlike RIP’s 15)
• Open standard (not proprietary like EIGRP)

OSPF Key Concepts

Areas

graph TD
    subgraph "Area 0 (Backbone)"
        ABR1[ABR 1]
        ABR2[ABR 2]
    end
    subgraph "Area 1"
        R1[Router 1]
        R2[Router 2]
    end
    subgraph "Area 2"
        R3[Router 3]
        R4[Router 4]
    end
    R1 --> ABR1
    R2 --> ABR1
    R3 --> ABR2
    R4 --> ABR2
    ABR1 --> ABR2

• Area 0 (Backbone): All areas must connect to Area 0
• ABR (Area Border Router): Connects areas to the backbone
• ASBR (Autonomous System Boundary Router): Connects OSPF to external networks
• Stub area: Doesn’t receive external routes (replaced with default route)
• NSSA (Not-So-Stubby Area): Stub area that can have an ASBR

Router Types

OSPF Neighbor States

stateDiagram-v2
    [*] --> Down
    Down --> Init: Hello received
    Init --> TwoWay: My RID in neighbor's Hello
    TwoWay --> ExStart: DR/BDR elected
    ExStart --> Exchange: Master/slave determined
    Exchange --> Loading: DBD exchange complete
    Loading --> Full: LSR/LSU/LSAck complete
    Full --> [*]: Adjacency established

OSPF Packet Types

LSA Types

OSPF Cost Calculation

Cost = Reference Bandwidth / Interface Bandwidth
Default Reference Bandwidth = 100 Mbps (10^8)

Problem: With default reference bandwidth, GigE and 10GigE have the same cost. Solution: increase reference bandwidth.

OSPF Network Types

OSPF Configuration (Cisco)

router ospf 1
  router-id 1.1.1.1
  network 192.168.1.0 0.0.0.255 area 0
  network 10.0.0.0 0.0.0.255 area 1
  passive-interface GigabitEthernet0/1
  default-information originate

SPF Algorithm Walkthrough

graph LR
    A((A)) -->|cost 2| B((B))
    A -->|cost 5| C((C))
    B -->|cost 3| C
    B -->|cost 1| D((D))
    C -->|cost 1| D

Dijkstra from A:

1. Start at A: {A: 0}
2. Neighbors: B(2), C(5)
3. Visit B (cost 2): C via B = 2+3=5 (no improvement), D = 2+1=3
4. Visit D (cost 3): Done
5. Visit C (cost 5): Done

Result: A→B(2), A→B→D(3), A→C(5)

Interview Questions

1. Q: Why does OSPF use areas? A: Areas limit the scope of LSA flooding, reduce LSDB size on each router, and speed up SPF calculations. Without areas, every topology change triggers SPF on every router.

2. Q: What is the difference between OSPF and RIP? A: OSPF is link-state (Dijkstra, fast convergence, unlimited hops, areas, VLSM). RIP is distance-vector (Bellman-Ford, slow convergence, 15-hop limit, classful in v1).

3. Q: What triggers OSPF SPF recalculation? A: Changes in Type 1 or Type 2 LSAs (router or network LSAs). Changes in Type 3-5 LSAs trigger partial route calculations, not full SPF.

4. Q: What is the DR/BDR election process? A: On multi-access networks (like Ethernet), OSPF elects a Designated Router (DR) and Backup DR (BDR) to reduce adjacencies. Election is based on: 1) Highest OSPF priority (0 = not eligible), 2) Highest Router ID. DR/BDR are not preemptive.

5. Q: What is OSPF virtual link? A: A virtual link connects a non-backbone area to Area 0 through another area. It’s a workaround for areas that can’t directly connect to the backbone. Configured between two ABRs.

6. Q: How does OSPF handle equal-cost multipath (ECMP)? A: OSPF installs multiple equal-cost paths in the routing table. By default, up to 4 (Cisco) or 16 paths are supported. Traffic is load-balanced across them.

Common Mistakes

• Forgetting that all areas must connect to Area 0
• Not matching Hello/Dead timers on neighbors
• Not matching area IDs on directly connected interfaces
• Confusing OSPF cost with hop count (it’s bandwidth-based)
• Forgetting DR/BDR election is not preemptive
• Using default reference bandwidth on networks with links faster than 100 Mbps

Summary

OSPF is the standard enterprise IGP. It uses Dijkstra’s algorithm, organizes routers into areas for scalability, elects DR/BDR on multi-access networks, and converges quickly. Key interview topics: areas, LSA types, neighbor states, DR/BDR, and SPF algorithm.

Cross-References

• Routing Overview
• RIP — Distance-vector comparison
• IS-IS — Another link-state IGP
• BGP — Exterior protocol
• Static vs Dynamic

Cross References

• BGP
• IS-IS
• RIP
• IP Protocol