Single-area OSPF for CCNA: neighbours, states and the faults that appear in simulations
Two OSPF routers become neighbours only if they agree on area, subnet, hello and dead timers and authentication. They move through Down, Init, 2-Way, ExStart, Exchange and Loading to Full. When a neighbour fails to appear, check interfaces, addressing, area, timers and any filter, in that order, and read show ip ospf neighbor.
OSPF looks like a long list of terms. In the exam it comes down to one question: why are these two routers not talking?
Where OSPF sits in v2.0
IP Routing carries 20 per cent of the v2.0 exam. Topic 3.3 names single-area OSPFv2 and OSPFv3, so the IPv6 version is examined directly as well. OSPF is also the routing topic candidates meet most often in simulations.
What OSPF does, in two sentences
Routers running OSPF discover neighbours, exchange a description of their links and each build the same map of the network. Every router then runs the same calculation on that map to choose its best paths.
When two routers become neighbours
For routers on a shared link to form a neighbour relationship, these must line up:
| Setting | Must be |
|---|---|
| Area ID | The same on both interfaces |
| Subnet and mask | In the same subnet, with matching masks |
| Hello and dead timers | The same on both ends |
| Authentication | The same, if used |
| Router ID | Different on each router |
A mismatch in any of the first four stops the neighbour from forming. A duplicate router ID causes odd behaviour that is worth recognising on sight.
The neighbour states
Learn them in order, because the state where a neighbour sticks is itself a clue.
- Down. No hellos received.
- Init. A hello was received, but the neighbour has not yet seen you.
- 2-Way. Both sides see each other. On broadcast links, routers that are not the designated router or backup remain here with each other, and that is normal.
- ExStart and Exchange. Routers agree who leads and swap summaries of their databases.
- Loading. They request the entries they lack.
- Full. Databases match.
A neighbour stuck in Init often means the other side is not hearing you, so look for a filter. A neighbour stuck in ExStart or Exchange is a classic sign of a mismatch in maximum transmission unit.
The router ID
OSPF chooses a router ID in this order: a manually set ID, the highest loopback address, then the highest active interface address. Setting it by hand makes every output easier to read and avoids surprises when interfaces change.
A checklist when no neighbour appears
- Is the interface up and up?
- Are the addresses and masks right, and in the same subnet?
- Is the interface in the OSPF process, with the same area on both sides?
- Do hello and dead timers match?
- Is an access list blocking OSPF messages?
- Is the interface passive where it should not be?
Read show ip ospf neighbor first, then show ip ospf interface for timers and area. Fix one thing at a time and check again.
Practising it
Build two routers joined by a link in Packet Tracer and form the neighbour. Then break it in each of the ways above and write down what the output looked like. That is more instructive than any diagram, and it practises the troubleshooting that v2.0 adds across the whole exam.
Do not overspend hours
Routing is 20 per cent of the exam, not 80. In a 150-hour plan, the starting split gives IP routing about 30 hours, and OSPF is only part of that. Static routes and the routing table deserve their share before you go deep into OSPF.
Spend an hour on it: form a neighbour, break it, fix it and move on to switching.
Questions readers ask
Is OSPFv3 on the CCNA v2.0 exam?
Why is my OSPF neighbour stuck in 2-Way?
Do I need to know multi-area OSPF for CCNA?
This article is independent and is not endorsed by Cisco. Facts change: confirm them on the official page before you act.