IP Connectivity // Lesson 04

Single-Area OSPFv2.

Configure and verify OSPFv2 neighbours, router IDs, point-to-point links and broadcast DR/BDR behaviour in one area.

OSPFv2Area 0AdjacencyDR/BDRRouter ID
01 // Mental model

Start with the big picture.

Routers advertise link state, build a common database and calculate shortest paths with SPF.

CCNA focus: Start troubleshooting at the interface: up/up, correct IP subnet, same area and compatible timers/network type. Then inspect neighbour state and learned O routes.
02 // Building blocks

Know what each part does.

01

Link-state IGP

Routers advertise link state, build a common database and calculate shortest paths with SPF.

02

Router ID

A 32-bit identifier selected explicitly or from interface addresses; it is not required to be reachable.

03

Adjacency

Neighbours must agree on key parameters such as area, timers, network type and authentication.

04

Network type

Broadcast links elect a DR and BDR; point-to-point links do not.

03 // Compare and recognise

Read the clues.

ItemWhat to remember
AreaInterfaces in the same adjacency must use the same OSPF area.
Passive interfaceAdvertises the connected network without sending hellos or forming neighbours there.
DR/BDRReduces adjacency overhead on multiaccess broadcast segments.
CostOSPF metric based primarily on interface bandwidth/reference bandwidth.
04 // Configure and verify

Configure and verify.

R1(config)# router ospf 10
R1(config-router)# router-id 1.1.1.1
R1(config-router)# passive-interface default
R1(config-router)# no passive-interface gigabitEthernet 0/1
R1(config)# interface gigabitEthernet 0/1
R1(config-if)# ip ospf 10 area 0
R1(config-if)# ip ospf network point-to-point
R1# show ip ospf neighbor
R1# show ip ospf interface brief

Read the example from top to bottom, then verify the resulting state. Configuration is only complete when the output matches the intended design.

05 // Exam and troubleshooting

Turn facts into a method.

  • Interfaces in the same adjacency must use the same OSPF area.
  • Advertises the connected network without sending hellos or forming neighbours there.
  • Reduces adjacency overhead on multiaccess broadcast segments.
  • OSPF metric based primarily on interface bandwidth/reference bandwidth.
Exam checkpoint: Start troubleshooting at the interface: up/up, correct IP subnet, same area and compatible timers/network type. Then inspect neighbour state and learned O routes.
06 // Check yourself

Single-Area OSPFv2 quiz.

1. Which algorithm does OSPF use to calculate shortest paths?

2. Where is a DR/BDR election expected?

3. What does passive-interface do in OSPF?

4. Which neighbour state indicates a complete adjacency?

5. Must an OSPF router ID be an active reachable IPv4 address?

Score: 0 / 5