CCNA OSPF: Configure Single-Area OSPFv2 and OSPFv3, Predict the DR, and Fix Neighbors Stuck Below Full

OSPF configuration on the CCNA is short: a few lines per router. What takes practice is predicting what those lines produce: which router becomes DR, which ID a router picks, and why two routers that can ping each other still refuse to become neighbors.

What CCNA v2.0 objective 3.3 covers on OSPF

The v2.0 exam topics, which apply from February 3, 2027, list objective 3.3: "Configure single area OSPFv2 for IPv4 and OSPFv3 for IPv6," with four sub-items: neighbor adjacencies (excluding authentication), point-to-point, broadcast (DR/BDR selection) and router ID. The v1.1 outline, in use through February 2, 2027, has the same four sub-items, without the authentication note, for OSPFv2 only (3.4). OSPFv3 is new in v2.0.

Two boundaries help you study efficiently. Everything is single-area, so multi-area design and area border routers aren't part of the objective. And authentication is explicitly excluded. OSPF routes also feed objective 3.1, reading the routing table, which the CCNA routing table guide covers.

Configuring single-area OSPFv2 on a Cisco router

An original example written by Passy: R1 connects to R2 over 10.0.12.0/30 on G0/0 and has a user LAN, 192.168.10.0/24, on G0/1.

interface GigabitEthernet0/0
 ip address 10.0.12.1 255.255.255.252
 ip ospf network point-to-point
!
router ospf 1
 router-id 1.1.1.1
 network 10.0.12.0 0.0.0.3 area 0
 network 192.168.10.0 0.0.0.255 area 0
 passive-interface GigabitEthernet0/1

What each line does:

• router ospf 1 starts OSPF process 1. The process ID only matters on this router; neighbors don't need the same number. The area number, 0 here, must match.
• network ... area 0 enables OSPF on every interface whose address falls in the range, written with a wildcard mask. The alternative is to enable OSPF on the interface itself with ip ospf 1 area 0.
• passive-interface stops OSPF hellos on the LAN, so no neighbor forms there, while the LAN subnet is still advertised.
• ip ospf network point-to-point on a link with only two routers skips the DR/BDR election. Set it on R2's end too, because mismatched network types are a known neighbor problem. Without it, Ethernet defaults to the broadcast network type.

Verify with show ip ospf neighbor, show ip ospf interface (area, network type, cost, timers, DR and BDR) and show ip route ospf.

One number to know: OSPF cost. Cisco's OSPF command reference says the metric is the reference bandwidth divided by the interface bandwidth, with a default reference of 100 Mbps. A cost can't go below 1, so a 100 Mbps link and a 1 Gbps link both cost 1 by default. auto-cost reference-bandwidth raises the reference; set the same value on every router. For a larger set of routing practice, 160 IP Routing questions are planned for full CCNA prep.

Configuring OSPFv3 for IPv6 in the same area

Cisco's OSPFv3 configuration example lists the prerequisites: enable IPv6 unicast routing and enable IPv6 on the interface. OSPFv3 is then enabled per interface, not with network statements. An original example for R1:

ipv6 unicast-routing
!
interface GigabitEthernet0/0
 ipv6 address 2001:db8:12::1/64
 ipv6 ospf 1 area 0
!
ipv6 router ospf 1
 router-id 1.1.1.1

Three points about OSPFv3 matter for the exam:

• The router ID is still a 32-bit dotted number. Cisco's example sets it with router-id under ipv6 router ospf. OSPFv3 borrows the ID from an IPv4 address when the router has one, so on a router with no IPv4 address at all you have to configure it.
• Neighbors talk over link-local addresses, so OSPFv3 routes in show ipv6 route ospf point to an FE80:: next hop plus an exit interface.
• Verification commands mirror OSPFv2: show ipv6 ospf neighbor, show ipv6 ospf interface and show ipv6 route ospf.

How OSPF chooses its router ID

Cisco's duplicate router ID document gives the order:

1. The router-id command, if configured.
2. Otherwise, the highest IP address on a loopback interface.
3. Otherwise, the highest IP address on the router's other active interfaces.

Once chosen, the router ID doesn't change until the OSPF process restarts or the router reloads. The command reference adds that a new router-id on a running process takes effect at the next reload or after clear ip ospf process. Router IDs must be unique: Cisco's troubleshooting guide says routers with the same router ID ignore each other's hellos and don't become adjacent.

DR and BDR selection on Ethernet segments

On broadcast networks such as Ethernet, routers elect a designated router (DR) and a backup (BDR). Cisco's command reference for ip ospf priority sets the rules:

• Highest interface priority wins. The range is 0 to 255 and the default is 1.
• A tie goes to the highest router ID.
• Priority 0 means never. A router with priority 0 can't become DR or BDR.
• Priority applies only to multiaccess networks, not point-to-point links.

Cisco's neighbor states document explains the result you see in show ip ospf neighbor. On broadcast media, a router becomes Full only with the DR and BDR and stays in 2-Way with every other router; on point-to-point links it becomes Full with its neighbor. So 2WAY/DROTHER between two non-DR routers is normal. The election is also not preemptive. RFC 2328, the OSPFv2 standard, says a router that finds a DR already on the network accepts it regardless of its own priority, so a router that joins later with a higher priority doesn't take over until the current DR or BDR goes away.

Reading OSPF neighbor states when something is wrong

Cisco's neighbor troubleshooting guide maps what you see to likely causes:

Cisco's table of typical causes also lists mismatched network types. On the MTU problem, Cisco notes that ip ospf mtu-ignore exists but recommends fixing the MTU instead. The hello and dead timers on Ethernet default to 10 and 40 seconds; Cisco's neighbor states document puts the dead interval at four times the hello interval by default.

When a state question trips you up, the explanation matters more than the score. You'll be able to have AI Rescue, planned for launch, explain why the neighbor state you picked was wrong.

Original CCNA OSPF practice scenarios

All four are written by Passy for practice, not taken from any Cisco exam.

1. The silent neighbor. R1 and R2 share 10.0.12.0/30 and ping each other. show ip ospf neighbor on both is empty. show ip ospf interface shows "Hello 5, Dead 20" on R1 and "Hello 10, Dead 40" on R2. Keyed answer: the timers don't match, so neither router accepts the other's hellos. Set the same values on both ends.

2. Who becomes DR? Four routers on one switch start OSPF together: R1 (priority 1, router ID 1.1.1.1), R2 (priority 50, 2.2.2.2), R3 (priority 1, 3.3.3.3) and R4 (priority 0, 4.4.4.4). Keyed answer: R2 is DR, because its priority is highest. R3 is BDR: R1 and R3 tie at priority 1, and 3.3.3.3 is the higher router ID. R4 never takes either role. Later, R5 joins with priority 100. Nothing changes, because the election doesn't preempt. If R2 then fails, R3 becomes DR and R5 wins the new BDR election.

3. The unexpected router ID. R3 has no router-id command, Loopback0 at 3.3.3.3, Loopback1 at 33.0.0.1 and G0/0 at 192.168.50.1. Keyed answer: 33.0.0.1, the highest loopback address. A loopback is preferred even though the physical interface has a higher address. Adding router-id 3.3.3.3 changes it only after clear ip ospf process or a reload.

4. Stuck in EXSTART. R1's G0/0 has an MTU of 1500 and R2's has 1400. The neighbors appear but never reach Full. Keyed answer: the MTU mismatch stalls the database exchange. Match the MTU on both interfaces.

Missed one of these on the first pass? Redemption challenges, planned for full CCNA prep, bring back the questions you missed so you can try them again.

Making OSPF stick before exam day

1. Lab both versions on the same three routers: one point-to-point link and one Ethernet segment with a switch, so you see both network types. The CCNA lab practice guide compares the free tools.
2. Break one parameter at a time: timers, area, mask, MTU, passive interface. Predict the neighbor state before you look.
3. Change priorities and reset the process to watch the DR move, and note when it doesn't.
4. Read the routes OSPF installs and check the next hop for a few destinations, using the same subnet math as the CCNA subnetting guide.

The 20 free CCNA practice questions include an OSPF election item with a full explanation. It's a quick check of whether the rules above are automatic yet.