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

Term Description
Router ID (RID) Unique 32-bit ID (often highest IP or manually set).
Area Logical 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 ID Locally significant ID to differentiate OSPF instances on a router.

✅ OSPF Network Types in Single Area

Type Description Example Use Case
Broadcast Supports multiple routers; uses DR/BDR election Ethernet LANs
Point-to-Point One router directly connects to another Serial or PPP links
NBMA No broadcast support; manual neighbor setup Frame 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 Mask Wildcard Mask Purpose
255.255.255.0 0.0.0.255 Used in OSPF ACLs, etc.
255.255.254.0 0.0.1.255 Used 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)

Timer Default Purpose
Hello 10 sec Interval between Hello packets
Dead 40 sec Time 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 Type Name Description
Type 1 Router LSA Lists links of the router in the area
Type 2 Network LSA Generated 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

Vendor Command Differences
Cisco IOS router ospf, network, area
Juniper Uses edit protocols ospf, and set area 0.0.0.0 interface
MikroTik Configured via WinBox or CLI under /routing ospf

✅ Basic Network Design for Single-Area OSPF

Topology Description
Hub-and-Spoke One central router, many branches
Full Mesh All routers interconnected
Linear Routers 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

Task Command/Description
Enable OSPF router ospf [process-id]
Assign networks to area network [IP] [wildcard] area [ID]
Set RID router-id [X.X.X.X]
Verify neighbor show ip ospf neighbor
Make interface passive passive-interface [int]
Advertise default route default-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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