ospf states

 

**🔄 OSPF has 7 states before two routers can exchange routes. Miss one step — and nothing works.**

Most engineers know OSPF forms neighbor relationships. But do you know exactly what happens between the moment two routers first see each other and the moment they're fully exchanging topology data?

Understanding these 7 states is what makes you dangerous at troubleshooting OSPF. Let's walk through them like a story.

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**1️⃣ Down — "I haven't heard anything yet"**

This is the starting point. No Hello packets have been received from the neighbor. The router knows nothing. This isn't necessarily a problem — it just means the process is beginning, or the neighbor has stopped responding and the Dead timer has expired.

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**2️⃣ Init — "I can see you, but you can't see me yet"**

A Hello packet has been received from a neighbor — but that neighbor's Hello doesn't include your Router ID yet. You've spotted them. They haven't spotted you back.

Think of it like waving at someone across the street. You see them. They haven't looked your way yet.


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**3️⃣ 2-Way — "We can see each other — we're neighbors"**

Now both routers have seen each other's Router IDs in their Hello packets. This is the point where a formal neighbor relationship is established.

This is also where the DR and BDR election happens on multi-access networks like Ethernet.
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**4️⃣ Exstart — "Who's in charge here?"**

The routers now decide who is Master and who is Slave based on the highest Router ID. The Master controls the sequence numbers for the upcoming DBD exchange.

This negotiation happens via empty DBD packets.

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**5️⃣ Exchange — "Here's my table of contents"**

Master and Slave established. Now the routers swap DBD (Database Description) packets — summaries of their Link State Databases. Neither router is sending full topology data yet, just the index.

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**6️⃣ Loading — "Send me what I'm missing"**

After comparing DBDs, each router knows what information it's missing. LSR (Link State Request) packets go out requesting the missing LSAs. The neighbor responds with LSU (Link State Update) packets carrying the actual data.

This is the state where the real topology information finally changes hands.


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**7️⃣ Full — "We're in sync. Let's route some traffic. ✅"**


**The 7 States at a Glance:**

🔴 Down — No Hellos received
🟡 Init — One-way Hello received
🟡 2-Way — Mutual Hellos, neighbors formed
🟠 Exstart — Master/Slave negotiation
🟠 Exchange — DBD summary exchange
🟠 Loading — Requesting missing LSAs
🟢 Full — Fully synchronized, routing
traffic