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Module - 1 Single Area Open Shortest Path First

2026-08-18 16:58 Update: 2026-08-26

Tags: #enterpriseNetwork

Author: Duke Hsu

Topic

  1. OSPF Features and Characteristics
  2. Components of OSPF
  3. Three OSPF Databases
  4. SPF Algorithm
  5. Link-State Operation
  6. Single-Area and Multi-Area OSPF
  7. OSPF Operational States
  8. Hello Packet
  9. DR / BDR / DROTHER
  10. OSPF Cost Calculation

1.0 OSPF Features and Characteristics

OSPF - Open Shortest Path First

1.1 Core concept

  • OSPF is a Link-State Routing Protocol
  • OSPF has faster convergence than RIP
  • OSPF is more scalable
  • OSPF user areas
  • Link-State information includes:
    • Network Prefix
    • Prefix Length
    • Cost

OSPF learns the network topology and calculates the shortest path.

2.0 Components of OSPF

OSPF routers exchange routing information with each other through OSPF messages.

OSPF has five packet types:

Type Packet name Description Memorize
1 Hello Discovers neighbors and builds adjacencies between them find other
2 Database Description(DBD) Checks for database synchronization between routers informed other
3 Link-State Request(LSR) Requests specific link-state records from router to router ask status
4 Link-State Update(LSU) Sends specifically requested link-state records reply status
5 Link-State Acknowledgment(LSAck) Acknowledges the other packet types noted
graph LR A[Hello] --> B[DBD] --> C[LSR] --> D[LSU] --> E[LSAck]

3.0 Three OSPF Databases

Database Table Command
Adjacency Database Neighbor table show ip ospf neighbor
Link-State Database(LSDB) Topology Table show ip ospf database
Forwarding Database Routing Table show ip route
graph LR A[Neighbor] --> B[Topology] --> C[RoutingTable]

4.0 SPF Algorithm

OSPF Path Selection

OSPF uses the Dijkstra Shortest Path First (SPF) Algorithm to calculate the best path.

OSPF uses Cumulative Cost to compare different paths.

Lower Cost = Better Path

Example:

Path A: R1 → R2 → R4 Cost: 10 + 10 = 20

Path B: R1 → R3 → R4 Cost: 5 + 10 = 15

Path B → Cost 15 ✅

graph TD A[Establish Neighbor Adjacencies] --> B[Exchange Link-State Advertisements] --> C[Build the Link-State Datase] --> D[Execute the SPF Algorithm] --> E[Choose the Best Route]

6.0 Single-Area and Multiarea OSPF

6.1 Single-Area OSPF

All routers are in the same OSPF area.

Usually, we use Area 0.

Example:

R1 ─── R2 ─── R3 Area 0

6.2 Multiarea OSPF

There are multiple OSPF areas.

All other areas must connect to the Backbone Area (Area 0).

A router that connects Area 0 to another area is called an:

ABR = Area Border Router

Example:

Area 1 Area 0 Area 2 R1 ─── [ABR] ─── R2 ─── [ABR] ─── R3

7.0 OSPF Operational States

Down → Init → Two-Way → ExStart → Exchange → Loading → Full

State 中文理解
Down 還沒收到 Hello
Init 收到對方 Hello
Two-Way 雙方已經互相認識
ExStart 決定誰先開始交換 DBD
Exchange 交換 DBD
Loading 用 LSR / LSU 補齊資訊
Full LSDB 完全同步

8.0 Hello Packet

Hello Packet

The default OSPFv2 Hello interval on multiaccess and point-to-point networks is 10 seconds, while the default Dead interval is 40 seconds.

The OSPF Hello Packet has several important functions:

  • Discover Neighbors

  • Discover other OSPF routers.

  • Establish Adjacency

  • Establish and maintain OSPF neighbor relationships.

  • Advertise Parameters

  • Exchange parameters required to become OSPF neighbors.

  • Elect DR and BDR

  • Elect a Designated Router (DR) and Backup Designated Router (BDR) on multiaccess networks.

Point-to-point links do not require DR/BDR election.

OSPFv2 Hello Multicast Address

OSPFv2 Hello packets use the following IPv4 multicast address:

224.0.0.5 = AllSPFRouters (All OSPF Routers)

Class note

- Use LEC module to in LAB / This week

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References