Single-Area OSPF Configuration – Detailed Explanation

✅ Introduction to Single-Area OSPF

Concept and Advantages:
Single-Area OSPF is a basic OSPF deployment where all routers belong to the same area, typically Area 0, also called the backbone area.

  • Simple to configure and manage.
  • Ideal for small to medium-sized networks.
  • Faster convergence due to fewer LSAs.
  • No need to deal with inter-area summarization.

✅ When and Where It Is Used

  • Campus networks.
  • Small branch office networks.
  • Test labs or classroom simulations.
  • Any network with fewer than 50 routers.

✅ Basic OSPF Terminology

TermDescription
Router ID (RID)Unique 32-bit ID (often highest IP or manually set).
AreaLogical grouping of routers; all routers in single-area use same area ID.
Autonomous System (AS)The entire OSPF domain under common administrative control.
OSPF Process IDLocally significant ID to differentiate OSPF instances on a router.

✅ OSPF Network Types in Single Area

TypeDescriptionExample Use Case
BroadcastSupports multiple routers; uses DR/BDR electionEthernet LANs
Point-to-PointOne router directly connects to anotherSerial or PPP links
NBMANo broadcast support; manual neighbor setupFrame Relay, ATM

✅ Enabling OSPF on Routers

Router(config)# router ospf 1
Router(config-router)# network 192.168.1.0 0.0.0.255 area 0

✅ Setting the Router ID

Manual:

Router(config-router)# router-id 1.1.1.1

Automatic (default): Highest loopback interface IP. If no loopback, highest active interface IP.

✅ Interface-Level OSPF Configuration (Alternative Method)

Router(config)# interface GigabitEthernet0/0
Router(config-if)# ip ospf 1 area 0

✅ Understanding and Calculating Wildcard Masks

Subnet MaskWildcard MaskPurpose
255.255.255.00.0.0.255Used in OSPF ACLs, etc.
255.255.254.00.0.1.255Used to match ranges

Wildcard = Inverse of subnet mask

✅ Verifying OSPF Configuration

Router# show ip ospf
Router# show ip ospf neighbor
Router# show ip ospf interface
Router# show ip route ospf

✅ Passive Interfaces in OSPF

Router(config-router)# passive-interface GigabitEthernet0/1

✅ Default Route Advertisement

Router(config)# ip route 0.0.0.0 0.0.0.0 192.168.1.1
Router(config)# router ospf 1
Router(config-router)# default-information originate

✅ OSPF Timers (Optional)

TimerDefaultPurpose
Hello10 secInterval between Hello packets
Dead40 secTime to declare neighbor down
Router(config-if)# ip ospf hello-interval 10
Router(config-if)# ip ospf dead-interval 40

✅ Must match on both sides for neighbor adjacency to form!

✅ LSA Types Used in Single-Area OSPF

LSA TypeNameDescription
Type 1Router LSALists links of the router in the area
Type 2Network LSAGenerated by DR on broadcast networks

✅ Troubleshooting Single-Area OSPF

  • Mismatched area IDs
  • Missing or incorrect wildcard masks
  • Router ID conflict
  • Interface not enabled for OSPF
Router# debug ip ospf adj
Router# ping [neighbor IP]
Router# traceroute [destination]

✅ Redistribution (if needed)

Router(config)# router ospf 1
Router(config-router)# redistribute static subnets

✅ Configuration on Multi-Vendor Platforms

VendorCommand Differences
Cisco IOSrouter ospf, network, area
JuniperUses edit protocols ospf, and set area 0.0.0.0 interface
MikroTikConfigured via WinBox or CLI under /routing ospf

✅ Basic Network Design for Single-Area OSPF

TopologyDescription
Hub-and-SpokeOne central router, many branches
Full MeshAll routers interconnected
LinearRouters connected one after another

✅ Sample Configuration – Cisco Example

R1(config)# router ospf 10
R1(config-router)# router-id 1.1.1.1
R1(config-router)# network 10.0.12.0 0.0.0.255 area 0
R1(config-router)# network 192.168.1.0 0.0.0.255 area 0

On another router:

R2(config)# router ospf 10
R2(config-router)# router-id 2.2.2.2
R2(config-router)# network 10.0.12.0 0.0.0.255 area 0

✅ Summary Table

TaskCommand/Description
Enable OSPFrouter ospf [process-id]
Assign networks to areanetwork [IP] [wildcard] area [ID]
Set RIDrouter-id [X.X.X.X]
Verify neighborshow ip ospf neighbor
Make interface passivepassive-interface [int]
Advertise default routedefault-information originate

Single-Area OSPF Configuration Quiz

1. What is the typical area ID used in Single-Area OSPF?

Correct answer is B. Single-area OSPF usually uses Area 0, the backbone area.

2. Which OSPF command assigns a router ID manually?

Correct answer is A. The router-id command under router ospf sets the manual router ID.

3. What is the purpose of a wildcard mask in OSPF network statements?

Correct answer is D. Wildcard masks specify which bits to ignore when matching network addresses in OSPF.

4. Which of the following is NOT a network type supported in single-area OSPF?

Correct answer is C. Mesh is a topology, not an OSPF network type.

5. How do you enable OSPF on a specific interface directly?

Correct answer is A. The ip ospf command on interface enables OSPF on that interface.

6. What is the default Hello interval in OSPF?

Correct answer is B. The default Hello interval is 10 seconds.

7. Which OSPF LSA type lists the router's links within an area?

Correct answer is D. Type 1 LSAs describe the router's own links.

8. What command verifies OSPF neighbor relationships?

Correct answer is C. This command lists the OSPF neighbors and their states.

9. Which OSPF configuration command propagates a default route within Area 0?

Correct answer is B. The default-information originate command advertises the default route.

10. What happens if mismatched Hello or Dead intervals exist between OSPF neighbors?

Correct answer is A. Mismatched timers prevent neighbor adjacency formation.

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