3.5 Single-Area OSPFv2 — Configuration

Cisco Certified Network Associate CCNA

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1 Introduction

About the CCNA 200-301 Exam Domains and Weights Study Strategies and Tips

2 Chapter 1: Network Fundamentals (20%)

1.1 Network Components 1.2 Network Topology Architectures 1.3 Physical Interfaces and Cabling 1.4 TCP vs. UDP 1.5 IPv4 Addressing and Subnetting 1.6 IPv6 Addressing and Prefix 1.7 Wireless Principles 1.8 Virtualization Fundamentals 1.9 Switching Concepts 1.10 OSI and TCP/IP Reference Models 1.11 Verifying IP Parameters on Client Operating Systems

3 Chapter 2: Network Access (20%)

2.1 VLANs — Configuration and Verification 2.2 Trunking and 802.1Q 2.3 Layer 2 Discovery Protocols (CDP and LLDP) 2.4 EtherChannel (Link Aggregation) 2.5 Spanning Tree Protocol (STP) and RSTP 2.6 Wireless Architectures and AP Modes 2.7 Network Access Troubleshooting 2.8 Inter-VLAN Routing — Deep Dive

4 Chapter 3: IP Connectivity (25%)

3.1 Routing Table Components 3.2 Router Forwarding Decisions 3.3 IPv4 and IPv6 Static Routing 3.4 Single-Area OSPFv2 — Concepts 3.5 Single-Area OSPFv2 — Configuration 3.6 First Hop Redundancy Protocols (FHRP) 3.7 Troubleshooting IP Connectivity 3.8 OSPF Troubleshooting Scenarios

5 Chapter 4: IP Services (10%)

4.1 NAT (Network Address Translation) 4.2 DHCP and DNS 4.3 NTP, SNMP, and Syslog 4.4 QoS Fundamentals 4.5 TFTP/FTP and Remote Access 4.6 IP Services — Comprehensive Review

6 Chapter 5: Security Fundamentals (15%)

5.1 Key Security Concepts 5.2 Device Access Control and Password Policies 5.3 VPN Fundamentals 5.4 Access Control Lists (ACLs) 5.5 Layer 2 Security Features 5.6 Network Security Best Practices and Hardening 5.7 Threat Mitigation and Incident Response

7 Chapter 6: Automation and Programmability (10%)

6.1 Impact of Automation on Network Management 6.2 Controller-Based and Software-Defined Architectures 6.3 AI and Machine Learning in Network Operations 6.4 REST-Based APIs 6.5 Configuration Management Tools 6.6 JSON-Encoded Data 6.7 Automation and Programmability — Comprehensive Review

3.5 Single-Area OSPFv2 — Configuration

Key Takeaways Explain This Topic AI

The OSPF Router ID is chosen from manual configuration, then highest loopback IP, then highest active interface IP.

The network command with a wildcard mask selects which interfaces OSPF activates.

Passive interfaces advertise their network but do not send or receive Hello packets.

On broadcast networks a DR and BDR are elected to reduce flooding and adjacency count.

DR election is highest OSPF priority (default 1, 0 excludes), then highest Router ID, and is non-preemptive.

Last updated: June 2026

OSPF Router ID

Every OSPF router needs a unique Router ID (RID) — a 32-bit value written like an IPv4 address.

Router ID Selection Order

Manually configured with the router-id command (highest priority).

Highest IP on any loopback interface, if no manual RID.

• Highest IP on any active physical interface, if no loopback exists. Router(config)# router ospf 1
• Router(config-router)# router-id 1.1.1.1 ! Best practice: set it manually

On the Exam: Changing the RID does not take effect until the process restarts with clear ip ospf process . Always set it manually so a future interface change cannot silently shift it.

• Basic OSPF Configuration Router(config)# router ospf 1 ! Process ID, locally significant
• Router(config-router)# router-id 1.1.1.1
• Router(config-router)# network 192.168.10.0 0.0.0.255 area 0 ! Activate matching interfaces
• Router(config-router)# network 10.0.0.0 0.0.0.3 area 0 ! Point-to-point /30 link
• Router(config-router)# passive-interface GigabitEthernet0/1 ! No Hellos toward the LAN
• Router(config-router)# auto-cost reference-bandwidth 10000 ! Differentiate Gigabit links
• Router(config-router)# default-information originate ! Advertise the default route

The process ID after router ospf is locally significant only — two routers can use different process IDs and still peer.

The network Command and Wildcard Masks

The network command tells OSPF which interfaces to run on. The wildcard mask is the inverse of the subnet mask (255 minus each octet):

• 255.255.255.0 → wildcard 0.0.0.255
• 255.255.255.252 → wildcard 0.0.0.3

• Command Effect
• network 192.168.10.0 0.0.0.255 area 0 Activates interfaces in 192.168.10.0/24
• network 10.0.0.0 0.0.0.3 area 0 Activates interfaces in 10.0.0.0/30
• network 0.0.0.0 255.255.255.255 area 0 Activates ALL interfaces in Area 0
• Alternative: Interface-Level OSPF Router(config)# interface GigabitEthernet0/0
• Router(config-if)# ip ospf 1 area 0 ! Enable OSPF directly on the interface

This approach is clearer and avoids wildcard-mask mistakes.

Passive Interfaces

A passive interface still has its network advertised into OSPF but stops sending and receiving Hellos, so no adjacency forms on it. Router(config-router)# passive-interface GigabitEthernet0/1 ! One interface

• Router(config-router)# passive-interface default ! All interfaces passive
• Router(config-router)# no passive-interface GigabitEthernet0/0 ! Re-enable specific ones

Use passive interfaces on LAN segments facing only end devices — there are no neighbors to find, and suppressing Hellos there is a security and stability best practice.

On the Exam: If two directly connected routers won't peer, check whether one side made the link passive. A passive interface silently kills the adjacency.

DR/BDR Election on Broadcast Networks

On broadcast multi-access networks (Ethernet), OSPF elects a Designated Router (DR) and Backup Designated Router (BDR) to limit flooding. Without them, N routers would build N×(N−1)/2 adjacencies; with a DR/BDR each router peers fully only with the DR and BDR.

Election Rules

Highest OSPF priority wins (default 1, range 0-255).

Tie → highest Router ID wins.

Priority 0 means the router never becomes DR or BDR.

The election is non-preemptive — a later, higher-priority router will not displace a sitting DR until the DR fails and a new election runs. Router(config-if)# ip ospf priority 100 ! Push this router to win

• Router(config-if)# ip ospf priority 0 ! Exclude from election
• Adjacency Types on Broadcast Networks

Relationship Reaches Full?

• DR ↔ every router Yes
• BDR ↔ every router Yes
• DROther ↔ DROther No — stays 2-Way

The DR receives LSAs on 224.0.0.6 and floods them on 224.0.0.5; the BDR shadows the DR and takes over on failure.

Point-to-Point Networks

On point-to-point links there are only two routers, so no DR/BDR election occurs. You can force Ethernet into this mode: Router(config-if)# ip ospf network point-to-point

This speeds adjacency formation and removes DR-failure concerns — a common optimization on back-to-back router links.

• Verification Commands Router# show ip ospf ! Process info, Router ID
• Router# show ip ospf neighbor ! Neighbor state (Full, 2-Way)
• Router# show ip ospf interface ! Per-interface cost, area, DR/BDR
• Router# show ip ospf interface brief
• Router# show ip ospf database ! The LSDB
• Router# show ip route ospf ! OSPF routes only
• Sample show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface
• 2.2.2.2 1 FULL/DR 00:00:35 10.0.0.2 Gi0/0
• 3.3.3.3 1 FULL/BDR 00:00:37 10.0.0.3 Gi0/0
• 4.4.4.4 1 2WAY/DROTHER 00:00:31 10.0.0.4 Gi0/0

Read each column: Neighbor ID is the neighbor's Router ID; Pri is its OSPF priority; State combines the adjacency state and the neighbor's DR/BDR role; Dead Time counts down and must stay above zero (it resets each Hello); and Address/Interface show where the neighbor lives. The 2WAY/DROTHER line is normal — that neighbor is neither DR nor BDR, so the local router intentionally stays at 2-Way with it.

Worked Example: Wildcard Mask Math

Wildcard masks trip up many candidates. To advertise the interface in 172.16.8.0/22, first find the subnet mask for /22: 255.255.252.0. The wildcard is the bit-inverse — subtract each octet from 255: 0.0.3.255. So the statement is network 172.16.8.0 0.0.3.255 area 0 . A quick check: the wildcard's last non-zero value (3) plus one equals the block size (4), confirming the /22 boundary. Getting the wildcard wrong either activates too many interfaces or none, so when a network statement seems to do nothing, recompute the wildcard first.

Choosing Network Statement vs. Interface Configuration

The classic network ... area method enables OSPF on every interface whose primary IP falls inside the wildcard range — convenient but easy to over-match. The modern interface method, ip ospf <process> area <area> applied directly under an interface, is explicit and avoids wildcard errors, which is why many shops and study guides now prefer it. Both produce identical OSPF behavior; the CCNA may show either, so recognize that a missing route can stem from an interface that no network statement happened to match, or from forgetting the per-interface ip ospf line.

Default-Route Origination

If the OSPF domain reaches the internet through one edge router that holds a static default route, that default is not shared with other OSPF routers automatically. The edge router must run default-information originate , which injects a 0.0.0.0/0 route into OSPF so every internal router learns a path to the gateway of last resort. Without it, internal routers can reach internal subnets but have no route off-net — a frequent "can ping locally, not the internet" scenario.