Single-Area OSPF Configuration – Complete Guide with Examples

1. What Is Single-Area OSPF?

Single-area OSPF places all routers in a single OSPF area — almost always Area 0 (the backbone). It is the simplest OSPF deployment: there are no Area Border Routers (ABRs), no inter-area summarisation, and the entire OSPF domain shares one Link-State Database (LSDB). Every router runs a full SPF calculation against the same topology.

  • Simple to configure and manage — only two LSA types (Type 1 and Type 2) are present
  • Fast convergence — fewer LSAs mean a smaller LSDB and quicker SPF runs
  • No inter-area summarisation overhead — no Type 3/4/5 LSAs to manage
  • Ideal for small to medium networks — up to roughly 50 routers before multi-area is recommended

Typical deployment scenarios: campus networks, small branch office networks, test labs, and any environment where all routers are under the same administrative control and the network diameter is manageable.

Related pages: OSPF Overview | OSPF Areas & LSAs | OSPF DR/BDR | OSPF Neighbor States | Wildcard Masks | Routing Metrics | Default Routes | show ip route | show ip protocols | Step-by-Step: OSPF Lab

2. Key OSPF Terminology

Term Description
Router ID (RID) Unique 32-bit identifier written in dotted-decimal (e.g., 1.1.1.1). Selected in order: manually configured → highest loopback IP → highest active interface IP. Must be unique across the OSPF domain.
OSPF Process ID Locally significant integer (1–65535) that identifies the OSPF instance on a router. Does not need to match between neighbouring routers.
Area ID 32-bit number written in decimal (e.g., 0) or dotted-decimal (e.g., 0.0.0.0). All routers sharing an adjacency must use the same area ID on that link.
Wildcard Mask Bitwise inverse of the subnet mask. Used in network statements. A ‘0’ bit = must match; a ‘1’ bit = ignored. Example: /24 subnet → wildcard 0.0.0.255. See: Wildcard Masks Explained
OSPF Cost Interface metric = Reference BW ÷ Interface BW. Default reference = 100 Mbps. Lower cost = preferred path. Raise the reference with auto-cost reference-bandwidth on modern networks.
LSDB Link-State Database — the topology map built from LSAs. All routers in the same area must have identical LSDBs before SPF can run.

3. OSPF Network Types in Single Area

Network Type DR/BDR? Hello / Dead Timers Description Example Links
Broadcast Yes 10 s / 40 s Multiple routers on same segment; DR/BDR elected to reduce adjacencies Ethernet LANs, Wi-Fi
Point-to-Point No 10 s / 40 s Exactly two routers; no DR/BDR; simplest and fastest adjacency formation Serial (HDLC/PPP), GRE tunnels
NBMA Yes 30 s / 120 s No broadcast support; manual neighbour configuration required Frame Relay hub-and-spoke, ATM
Point-to-Multipoint No 30 s / 120 s Each connection treated as a separate point-to-point; avoids DR/BDR complexity Frame Relay partial-mesh

4. Enabling OSPF – Two Methods

Method A: Network Statement (Global Config Mode)

router ospf 1
 router-id 1.1.1.1
 network 192.168.1.0 0.0.0.255 area 0   ! matches any interface in 192.168.1.0/24
 network 10.0.12.0 0.0.0.3 area 0       ! matches the /30 link to R2 precisely

The network statement activates OSPF on any interface whose IP falls within the specified range. Use the most specific wildcard possible to avoid accidentally enabling OSPF on unintended interfaces.

Method B: Interface-Level (More Precise — Preferred)

interface GigabitEthernet0/0
 ip ospf 1 area 0    ! enables OSPF process 1 directly on this interface

Method B is the preferred approach on modern Cisco IOS and IOS-XE. It is explicit, readable, and eliminates the risk of a broad wildcard mask activating OSPF on unintended interfaces.

5. Router ID Configuration

Always set the Router ID manually. If OSPF determines the RID automatically and an interface IP later changes or goes down, the RID may change — resetting all adjacencies and causing a full SPF recalculation. 

! Set Router ID manually (under router ospf)
router ospf 1
 router-id 1.1.1.1

! After changing the RID on a live router, reset the OSPF process to apply it:
Router# clear ip ospf process

! Verify the active RID:
Router# show ip ospf | include Router ID
! Output: Router ID 1.1.1.1

Automatic RID selection order (when not manually set): highest IP on any loopback interface → if no loopback, highest IP on any active physical interface. A dedicated loopback with a /32 mask is a stable RID source if manual configuration is not feasible.

6. Wildcard Mask Calculation

A wildcard mask is the bitwise inverse of the subnet mask. Subtract each octet of the subnet mask from 255 to get the wildcard.

Prefix Subnet Mask Wildcard Mask OSPF network statement example
/32 255.255.255.255 0.0.0.0 network 1.1.1.1 0.0.0.0 area 0 — matches exactly one IP (loopback)
/30 255.255.255.252 0.0.0.3 network 10.0.12.0 0.0.0.3 area 0 — matches 10.0.12.0–10.0.12.3
/24 255.255.255.0 0.0.0.255 network 192.168.1.0 0.0.0.255 area 0 — matches /24 range
/23 255.255.254.0 0.0.1.255 network 10.1.0.0 0.0.1.255 area 0 — matches 10.1.0.0–10.1.1.255
/16 255.255.0.0 0.0.255.255 network 10.0.0.0 0.0.255.255 area 0 — matches entire 10.0.x.x range

7. OSPF Cost and Reference Bandwidth

OSPF cost = Reference Bandwidth ÷ Interface Bandwidth. The default reference bandwidth is 100 Mbps, which means GigabitEthernet (1000 Mbps) and FastEthernet (100 Mbps) both calculate to cost 1 — making them indistinguishable to OSPF. Always raise the reference bandwidth on every router in the domain. 

! Raise reference bandwidth to 10 Gbps on ALL routers in the domain
! (must be consistent; change causes SPF recalculation)
router ospf 1
 auto-cost reference-bandwidth 10000   ! 10GigE=cost 1; 1GigE=cost 10; 100Mbps=cost 100

! Override cost on a specific interface (takes precedence over auto-cost)
interface GigabitEthernet0/1
 ip ospf cost 5

! Verify per-interface OSPF cost and timers
Router# show ip ospf interface GigabitEthernet0/1

8. Passive Interfaces

A passive interface still has its network advertised into OSPF, but Hello packets are suppressed — so no OSPF neighbours can form on that port. Use it on LAN-facing ports connected to end hosts, not to other routers. 

! Suppress Hellos on a specific interface
router ospf 1
 passive-interface GigabitEthernet0/1

! Best practice: make all interfaces passive by default, then selectively enable
router ospf 1
 passive-interface default
 no passive-interface GigabitEthernet0/0   ! enable Hellos toward R2
 no passive-interface GigabitEthernet0/2   ! enable Hellos toward R3

9. Default Route Advertisement

! Step 1: Static default route toward the Internet
ip route 0.0.0.0 0.0.0.0 203.0.113.1

! Step 2: Redistribute it into OSPF as a Type 5 External LSA
router ospf 1
 default-information originate

! Optional: always advertise even if no local default route exists
router ospf 1
 default-information originate always

Without always, the default is only advertised when a default route exists in the routing table. The always keyword is useful for edge routers that must continuously inject a default, but use it carefully — it can black-hole traffic if the upstream link goes down. See also: Default Routes

10. OSPF Timers

Hello and Dead intervals must match exactly on both sides of an adjacency. A mismatch prevents the neighbour relationship from forming and is one of the most common OSPF troubleshooting scenarios.

Timer Default (Broadcast / P2P) Default (NBMA) Purpose
Hello interval 10 seconds 30 seconds How often Hello packets are sent to maintain neighbour relationships
Dead interval 40 seconds (4 × Hello) 120 seconds Time without a Hello before the neighbour is declared down 
! Change timers on an interface (must match the remote side)
interface GigabitEthernet0/0
 ip ospf hello-interval 5
 ip ospf dead-interval 20

11. LSA Types in Single-Area OSPF

LSA Type Name Generated By Description Flooding Scope
Type 1 Router LSA Every OSPF router Describes the router’s own links, their states, and costs within the area Within originating area only
Type 2 Network LSA DR on multi-access (Ethernet) segments Describes the multi-access segment and all OSPF routers attached to it; only present when a DR exists Within originating area only

In single-area OSPF you will only ever see Type 1 and Type 2 LSAs unless external routes are redistributed (which generates Type 5 External LSAs from an ASBR). Type 3, 4, and 5 LSAs only appear in multi-area designs. See also: OSPF Areas & LSAs – Complete Guide

12. Verification Commands

! Confirm OSPF is running: shows Router ID, area, and SPF statistics
Router# show ip ospf

! List all OSPF neighbours and their adjacency state (goal: FULL)
Router# show ip ospf neighbor

! Show per-interface OSPF detail: cost, timers, DR/BDR, Hello/Dead intervals
Router# show ip ospf interface GigabitEthernet0/0

! Show routes learned via OSPF (marked O in the routing table)
Router# show ip route ospf

! Sample routing table OSPF entry:
!      O  10.2.0.0/24 [110/2] via 10.0.12.2, 00:05:33, GigabitEthernet0/0
!                      ^^^ ^
!                      AD  cost (sum of outgoing interface costs)

! Show the full Link-State Database
Router# show ip ospf database

13. Troubleshooting Single-Area OSPF

Problem Likely Cause Resolution
No neighbour in show ip ospf neighbor Mismatched area ID; Hello/Dead mismatch; ACL blocking OSPF (IP protocol 89); passive-interface on wrong port; stub flag mismatch Verify area IDs, timers; use debug ip ospf hello to see received Hellos
Neighbour stuck in ExStart or Exchange MTU mismatch; duplicate Router IDs on the segment Match MTUs or add ip ospf mtu-ignore; set unique RIDs with router-id
Route missing from routing table Interface not matched by network statement; passive-interface set incorrectly; wildcard too narrow Check show ip ospf interface; confirm all intended interfaces show OSPF enabled
Wrong path selected Default reference bandwidth too low (GigE and FastEthernet both = cost 1) Set auto-cost reference-bandwidth consistently on all routers; or tune with ip ospf cost
Router ID conflict Two routers share the same RID; adjacency instability results Set unique router-id on each router then clear ip ospf process 
! Debug adjacency events (use with caution in production)
Router# debug ip ospf adj

! Test basic reachability to neighbour
Router# ping 10.0.12.2

! Trace path to verify routing
Router# traceroute 10.2.0.1

14. Complete Two-Router Configuration Example

  Internet
      |
  [R1] Gi0/0: 10.0.12.1/30 ——— Gi0/0: 10.0.12.2/30 [R2]
   |                                                     |
  Gi0/1: 192.168.1.0/24                         Gi0/1: 172.16.1.0/24
  (passive, LAN)                                (passive, LAN)
            
! ===== R1 =====
router ospf 10
 router-id 1.1.1.1
 auto-cost reference-bandwidth 10000
 passive-interface default
 no passive-interface GigabitEthernet0/0   ! router link to R2
 network 10.0.12.0 0.0.0.3 area 0
 network 192.168.1.0 0.0.0.255 area 0
 default-information originate             ! advertise default route to R2

ip route 0.0.0.0 0.0.0.0 203.0.113.1      ! static default to Internet


! ===== R2 =====
router ospf 10
 router-id 2.2.2.2
 auto-cost reference-bandwidth 10000
 passive-interface default
 no passive-interface GigabitEthernet0/0   ! router link to R1
 network 10.0.12.0 0.0.0.3 area 0
 network 172.16.1.0 0.0.0.255 area 0


! ===== Verification on R2 =====
R2# show ip ospf neighbor
! Neighbor ID   Pri  State  Dead Time  Address      Interface
! 1.1.1.1         1  FULL/- 00:00:38   10.0.12.1    GigabitEthernet0/0

R2# show ip route ospf
! O*E2 0.0.0.0/0      [110/1] via 10.0.12.1 (default route from R1)
! O    192.168.1.0/24 [110/2] via 10.0.12.1 (R1 LAN)

15. Summary Command Reference

Task Command Mode
Enable OSPF process router ospf <process-id> Global config
Set Router ID manually router-id <X.X.X.X> Router OSPF config
Assign network to area network <IP> <wildcard> area <ID> Router OSPF config
Enable OSPF on interface (alternate) ip ospf <process-id> area <ID> Interface config
Raise reference bandwidth auto-cost reference-bandwidth <Mbps> Router OSPF config
Override interface cost ip ospf cost <value> Interface config
Suppress Hellos on interface passive-interface <int> Router OSPF config
Advertise default route default-information originate Router OSPF config
Verify neighbours show ip ospf neighbor Privileged EXEC
Verify interface details / cost show ip ospf interface <int> Privileged EXEC
Show OSPF-learned routes show ip route ospf Privileged EXEC
Show LSDB show ip ospf database Privileged EXEC

16. Key Points & CCNA Exam Tips

  • Single-area OSPF uses Area 0; only Type 1 (Router) and Type 2 (Network) LSAs are present unless redistribution adds Type 5 External LSAs
  • Enable with router ospf <process-id> then network <IP> <wildcard> area 0, or use interface-level ip ospf <pid> area 0
  • Wildcard mask = inverse of subnet mask; ‘0’ bits must match, ‘1’ bits are ignored
  • Always set router-id manually — automatic RID selection can change when interfaces go up/down, resetting all adjacencies
  • Always set auto-cost reference-bandwidth to match your fastest link; the default 100 Mbps makes GigE and FastEthernet both calculate to cost 1
  • passive-interface stops OSPF Hellos on a port while still advertising its network into OSPF
  • default-information originate advertises a default route as a Type 5 External LSA; requires a local default route unless always is added
  • Hello / Dead timers must match on both sides; default 10 s / 40 s on Ethernet and P2P; 30 s / 120 s on NBMA
  • OSPF process IDs are locally significant and do not need to match between routers
  • Verify adjacency with show ip ospf neighbor; the goal state is FULL
  • OSPF routes appear as [110/cost] in the routing table — AD = 110, second number = cumulative interface cost along the path

Single-Area OSPF Configuration Quiz

1. What area ID is typically used in single-area OSPF?

Correct answer is B. Single-area OSPF uses Area 0, the backbone area. All routers share one LSDB and run SPF on the same topology. There are no ABRs or inter-area LSAs.

2. Which command manually assigns an OSPF Router ID?

Correct answer is A. router-id 1.1.1.1 entered under router ospf sets the Router ID manually. This is best practice to prevent unexpected RID changes when interface IPs change, which would reset all adjacencies and trigger a full SPF recalculation.

3. What is the purpose of the wildcard mask in an OSPF network statement?

Correct answer is D. The wildcard mask is the bitwise inverse of the subnet mask. A ‘0’ bit must match; a ‘1’ bit is ignored. network 192.168.1.0 0.0.0.255 area 0 enables OSPF on any interface with an IP in the 192.168.1.0/24 range.

4. Which of the following is NOT a valid OSPF network type?

Correct answer is C. Mesh is a physical/logical topology, not an OSPF network type. The OSPF network types are Broadcast, Point-to-Point, NBMA, Point-to-Multipoint, and Point-to-Multipoint Non-Broadcast.

5. How do you enable OSPF on a specific interface using the interface-level method?

Correct answer is A. ip ospf 1 area 0 entered in interface configuration mode enables OSPF process 1 directly on that interface. This is the preferred method because it is explicit and avoids the risk of an overly broad wildcard mask activating OSPF on unintended interfaces.

6. What is the default Hello interval on Ethernet (broadcast) OSPF interfaces?

Correct answer is B. The default Hello interval on broadcast (Ethernet) and point-to-point interfaces is 10 seconds, with a Dead interval of 40 seconds (4×Hello). On NBMA interfaces the defaults are 30 s / 120 s. Both timers must match exactly on both sides for an adjacency to form.

7. Which OSPF LSA type describes a router’s own links within an area?

Correct answer is D. Every OSPF router generates a Type 1 Router LSA describing its own interfaces and their costs. In single-area OSPF you will only see Type 1 and Type 2 LSAs. Type 3 (Summary), Type 4 (ASBR Summary), and Type 5 (External) only appear in multi-area designs or when redistribution is configured.

8. Which command verifies OSPF neighbour relationships and their adjacency state?

Correct answer is C. show ip ospf neighbor lists all OSPF neighbours with their current adjacency state (goal: FULL), DR/BDR role, dead timer, and interface. This is the first command to run when troubleshooting OSPF.

9. Which OSPF command advertises a default route to all OSPF neighbours?

Correct answer is B. default-information originate under router ospf redistributes the default route (0.0.0.0/0) as a Type 5 External LSA to all OSPF neighbours. A static default route must already exist in the routing table unless the always keyword is appended.

10. What happens if Hello or Dead interval timers are mismatched between OSPF neighbours?

Correct answer is A. Hello and Dead timers must match exactly on both sides. A mismatch is detected during the Hello exchange and the neighbour state never advances past Init. Use show ip ospf interface to confirm and compare the timer values on each router.

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