OSPF Single-Area Basics

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Plain-English explanation

OSPF (Open Shortest Path First) is a link-state IGP. Routers describe their links in LSAs, build a common topology map, and run Dijkstra SPF to compute best paths. Single-area OSPF (Area 0 only) is the CCNA focus — all routers in one backbone area.

OSPF chooses routes by cost (bandwidth-based). Lower cost wins. AD is 110.

Deep dive

OSPF States — OSPF neighbor state machine — Down through Full.
From study charts · jdepew88 CCNA notes

OSPF Adj — OSPF neighbor adjacency table fields.
From study charts · jdepew88 CCNA notes

OSPF Packet Anatomy - 24 Octets — OSPF packet header — 24-byte common header.
From study charts · jdepew88 CCNA notes

OSPF Path Cost — OSPF path cost by interface bandwidth.
From study charts · jdepew88 CCNA notes

Router ID (RID): 32-bit dotted decimal — highest loopback IP, else highest active interface IP, else manual router-id.

Neighbors: Must match area, hello/dead timers, subnet on link, and authentication if used. State FULL = adjacency complete.

DR/BDR on Ethernet: Reduces LSA flooding on multi-access — know roles for exam; point-to-point links skip DR election.

Network types (awareness): broadcast, point-to-point, non-broadcast — CCNA focuses on broadcast and P2P.

Step-by-step — minimal single-area config

Three routers, all interfaces in Area 0:

router ospf 1
 router-id 1.1.1.1
 network 10.0.0.0 0.255.255.255 area 0

Modern alternative per interface:

interface GigabitEthernet0/0
 ip ospf 1 area 0
 ip ospf cost 10

Verify neighbors FULL, routes appear as O.

Commands to know

OSPF configuration

router ospf 1 router-id 2.2.2.2 passive-interface default no passive-interface GigabitEthernet0/0 network 192.168.1.0 0.0.0.255 area 0

OSPF verification

show ip ospf neighbor show ip ospf interface show ip route ospf show ip protocols

Troubleshooting

| Issue | Check | |-------|-------| | No neighbor | Timers, area mismatch, ACL blocking 89, wrong subnet | | Stuck in EXSTART/2-WAY | MTU mismatch, duplicate RID | | No OSPF routes | Network statement doesn't cover interface, passive interface | | Suboptimal path | Manual cost, reference bandwidth |

show ip ospf neighbor is your first command — no FULL, no LSDB sync.

Exam relevance

Exam trap

All normal OSPF areas connect to Area 0 (backbone). CCNA single-area means everything in Area 0 — still know Area 0 name.

Wildcard in network statement

OSPF network uses wildcard mask, not subnet mask — same as ACL rules from Week 2.

Practice checklist

Configure OSPF on three-router lab in Area 0 only
Set manual router-id and verify with show commands
Explain DR/BDR purpose in one sentence
Read OSPF costs from show ip ospf interface
Draw neighbor states from DOWN to FULL (high level)

What is OSPF's default administrative distance?

Which OSPF neighbor state indicates a completed adjacency?

Learning objectives