> ## Content Index > Fetch the complete content index at: https://www.pinglabz.com/llms.txt > Use this file to discover other available public pages before exploring further. # OSPF Neighbor States Explained (With Packet Exchange Examples) - URL: https://www.pinglabz.com/ospf-neighbor-states-explained/ - Published: 2026-03-24T03:16:24.000Z - Updated: 2026-08-01T19:31:24.000Z - Description: OSPF neighbors progress through 8 states from Down to Full, and understanding the packet exchange at each step is the key to troubleshooting adjacency problems. A guide to OSPF neighbor states with real packet captures. - Author: Jaime - Tags: OSPF, #Import 2026-08-01 19:54 ## The 8 OSPF Neighbor States ### 1\. Down **What it means:** No Hello packets have been received from the neighbor. This is the starting state. New to OSPF? Start with the [full OSPF guide](https://www.pinglabz.com/ospf/), then come back here for the deep dive. **What's happening:** - The router is either: - Waiting for the first Hello from a neighbor, or - No longer receiving Hellos (neighbor went down) **What you'll see:** You usually won't see "Down" in `show ip ospf neighbor` because down neighbors aren't listed. The absence of a neighbor means it's in the Down state. **Next state:** Init (when the first Hello is received) ### 2\. Init **What it means:** A Hello packet has been received from the neighbor, but this router's Router ID is **not yet listed** in the neighbor's Hello packet. **What's happening:** - You've heard from the neighbor - The neighbor hasn't heard from you yet (or hasn't acknowledged you) **Why you see this state:** - One-way communication (ACLs, firewall blocking return traffic) - Wrong subnet mask (router receives Hello but can't respond properly) - Authentication mismatch **What you'll see:** ``` Router# show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 192.168.1.2 1 INIT/DROTHER 00:00:33 192.168.1.2 Gi0/0 ``` **Troubleshooting:** If a neighbor is stuck in **Init**, there's one-way communication. Check: - ACLs blocking OSPF packets (protocol 89) - Subnet mask mismatch - Passive interface configuration **Next state:** 2-Way **Learn more:** [OSPF Neighbors Not Forming Troubleshooting](https://www.pinglabz.com/troubleshoot-ospf-neighbors-not-forming/) ### 3\. 2-Way **What it means:** Bidirectional communication has been established. Each router sees its own Router ID in the other's Hello packet. **What's happening:** - Hello packets are being exchanged successfully - On **point-to-point** links, routers immediately move to **ExStart** - On **multi-access** (Ethernet) networks, routers wait here while the **DR/BDR election** happens **Why this state is important:** On Ethernet, not all neighbors will progress past 2-Way. Only the DR and BDR will form full adjacencies with all routers. Regular routers (called "DROthers") stay at 2-Way with each other. **What you'll see:** ``` Router# show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 192.168.1.2 1 2WAY/DROTHER 00:00:34 192.168.1.2 Gi0/0 ``` **Key point:** **2-Way is normal** on multi-access networks if the router is not the DR or BDR. It's **not** normal on point-to-point links (it should quickly become Full). **Next state:** ExStart (if forming full adjacency) ### 4\. ExStart **What it means:** Routers are negotiating who will be the "master" and who will be the "slave" in the database exchange process. **What's happening:** - Routers exchange **empty DBD (Database Description)** packets with the **I (Initial), M (More), MS (Master/Slave)** bits set - The router with the **higher Router ID** becomes the master - The master controls the DBD sequence number **Why it matters:** This state is usually very brief (milliseconds to a few seconds). If neighbors get stuck here, it's almost always an **MTU mismatch**. **What you'll see:** ``` Router# show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 192.168.1.2 1 EXSTART/DR 00:00:35 192.168.1.2 Gi0/0 ``` **Troubleshooting:** If stuck in **ExStart**, check for **MTU mismatch**: ``` Router# show interface gi0/0 | include MTU MTU 1500 bytes ``` **Learn more:** [OSPF MTU Mismatch Troubleshooting](https://www.pinglabz.com/ospf-mtu-mismatch-troubleshooting/) **Next state:** Exchange ### 5\. Exchange **What it means:** Routers are exchanging **Database Description (DBD)** packets, which contain summaries of the LSAs in each router's LSDB. **What's happening:** - Each router sends DBD packets listing the LSAs it knows about - Routers compare the summaries to their own LSDB - If a router sees an LSA it doesn't have (or an LSA with a newer sequence number), it marks it for download **Why it matters:** This is where routers figure out what LSAs they need to request. **What you'll see:** ``` Router# show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 192.168.1.2 1 EXCHANGE/BDR 00:00:36 192.168.1.2 Gi0/0 ``` **Troubleshooting:** If stuck in **Exchange**, suspect: - MTU mismatch (DBD packets too large to send) - Overloaded router (CPU maxed out) **Next state:** Loading (if missing LSAs) or Full (if LSDBs are identical) ### 6\. Loading **What it means:** Routers are requesting and receiving missing LSAs. **What's happening:** - Router sends **Link-State Request (LSR)** packets asking for specific LSAs - Neighbor responds with **Link-State Update (LSU)** packets containing the LSAs - Router sends **Link-State Acknowledgment (LSAck)** to confirm receipt **Why it matters:** This is the state where LSDBs are synchronized. **What you'll see:** ``` Router# show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 192.168.1.2 1 LOADING/DR 00:00:37 192.168.1.2 Gi0/0 ``` **How long it lasts:** Usually very brief (seconds). In large networks with thousands of LSAs, it can take longer. **Next state:** Full ### 7\. Full **What it means:** The adjacency is fully formed. LSDBs are synchronized, and routers are exchanging LSA updates as needed. **What's happening:** - Routers have identical LSDBs (for the shared area) - SPF has been calculated - Routes are installed in the routing table - Routers will continue to exchange periodic Hellos and LSA updates **Why it matters:** **Full is the goal.** This is what you want to see on all adjacencies (except DROther-to-DROther on multi-access networks). **What you'll see:** ``` Router# show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 192.168.1.2 1 FULL/DR 00:00:38 192.168.1.2 Gi0/0 192.168.1.3 1 FULL/BDR 00:00:34 192.168.1.3 Gi0/0 192.168.1.4 0 2WAY/DROTHER 00:00:39 192.168.1.4 Gi0/0 ``` **Key observations:** - Neighbor `192.168.1.2` is the DR - Full adjacency - Neighbor `192.168.1.3` is the BDR - Full adjacency - Neighbor `192.168.1.4` is a DROther - 2-Way state (normal!) ### 8\. Attempt (Rare) **What it means:** Used only on **NBMA (Non-Broadcast Multi-Access)** networks when manually configured neighbors haven't responded. **What's happening:** The router is trying to send unicast Hellos to a manually configured neighbor, but hasn't received a response. **Where you'll see it:** Almost never, unless you're working with Frame Relay or manually configured NBMA neighbors. **What you'll see:** ``` Router# show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 10.1.1.2 1 ATTEMPT/DROTHER - 10.1.1.2 Serial0/0 ``` ## OSPF Neighbor State Transition Summary Here's the normal flow for a successful adjacency: **Down** What's HappeningNo Hellos received Packets ExchangedNone **Init** What's Happening Hello received, waiting for bidirectional Packets ExchangedHello **2-Way** What's Happening Bidirectional communication established Packets ExchangedHello **ExStart** What's Happening Master/slave negotiation Packets Exchanged Empty DBD (I, M, MS bits set) **Exchange** What's Happening LSA summaries exchanged Packets ExchangedDBD packets **Loading** What's HappeningMissing LSAs requested Packets ExchangedLSR, LSU, LSAck **Full** What's HappeningLSDBs synchronized Packets Exchanged Periodic Hellos, LSA updates **Point-to-Point Flow:** ``` Down → Init → 2-Way → ExStart → Exchange → Loading → Full ``` **Broadcast (Ethernet) Flow (DR/BDR):** ``` Down → Init → 2-Way → ExStart → Exchange → Loading → Full ``` **Broadcast (Ethernet) Flow (DROther-to-DROther):** ``` Down → Init → 2-Way (STOPS HERE) ``` ## What "FULL/DR," "FULL/BDR," and "2WAY/DROTHER" Mean When you run `show ip ospf neighbor`, you see two pieces of information in the State column: **Format:** `[State]/[Role]` ### Examples: **FULL/DR** - **State:** Full adjacency - **Role:** Neighbor is the Designated Router **FULL/BDR** - **State:** Full adjacency - **Role:** Neighbor is the Backup Designated Router **2WAY/DROTHER** - **State:** 2-Way (bidirectional communication, but no full adjacency) - **Role:** Neighbor is neither DR nor BDR **FULL/-** - **State:** Full adjacency - **Role:** Point-to-point link (no DR/BDR) ## When to Expect Full Adjacencies vs 2-Way ### Full Adjacencies Form: ✅ On **point-to-point** links (e.g., WAN serial links, GRE tunnels) ✅ Between **all routers and the DR** ✅ Between **all routers and the BDR** ✅ On **point-to-multipoint** networks ### 2-Way is Normal: ✅ Between **DROthers** on multi-access networks (Ethernet, etc.) **Example:** On an Ethernet segment with 5 routers: - 1 router becomes the DR - 1 router becomes the BDR - 3 routers are DROthers **Adjacencies:** - DR ↔ All routers: **Full** (4 adjacencies) - BDR ↔ All routers: **Full** (4 adjacencies) - DROther ↔ DROther: **2-Way** (no full adjacency) This design reduces overhead. Without DR/BDR, 5 routers would need **10 full adjacencies** (N × (N-1) / 2). With DR/BDR, only **8**. **Learn more:** [OSPF DR and BDR Explained](https://www.pinglabz.com/ospf-dr-bdr-designated-router/) Get the OSPF Field Reference - 9 pages, free Everything you'd want to remember about OSPF on nine printable pages. State machine diagram, LSA types, troubleshooting decision tree, copy-paste IOS XE templates, and real lab captures. Free for PingLabz members - just sign up with your email. [Get the OSPF cheat-sheet](https://www.pinglabz.com/ospf-cheatsheet/) ## Troubleshooting Stuck Neighbor States ### Stuck in Init **Problem:** One-way communication **Common causes:** - ACL blocking OSPF (IP protocol 89) - Firewall dropping OSPF packets - Subnet mask mismatch - Interface configured as passive **How to verify:** ``` Router# debug ip ospf adj OSPF-1 ADJ Gi0/0: Rcv hello from 192.168.1.2 area 0 OSPF-1 ADJ Gi0/0: 2-Way communication to 192.168.1.2 on Gi0/0, state 2WAY ``` If you don't see "2-Way communication," the neighbor isn't seeing your Hellos. That is the useful part: the router has already told you which direction is dead, so you only have to explain one side of the link. [Diagnosing a one-way OSPF hello](https://www.pinglabz.com/ospf-stuck-in-init-one-way-hello/) takes that reading and works through the ACL, the mask and the passive-interface cases one at a time. **Learn more:** [OSPF Neighbors Not Forming](https://www.pinglabz.com/troubleshoot-ospf-neighbors-not-forming/) ### Stuck in 2-Way (Point-to-Point Link) **Problem:** Should progress to ExStart, but doesn't **Common causes:** - Interface configured as broadcast instead of point-to-point - Misconfigured network type **How to check:** ``` Router# show ip ospf interface gi0/0 | include Network Type Process ID 1, Router ID 10.0.0.1, Network Type BROADCAST ``` **How to fix:** ``` Router(config)# interface gi0/0 Router(config-if)# ip ospf network point-to-point ``` **Learn more:** [OSPF Network Types Explained](https://www.pinglabz.com/ospf-network-types-explained/) ### Stuck in ExStart or Exchange **Problem:** MTU mismatch **Common causes:** - One router has MTU 1500, the other has MTU 1400 - Jumbo frames enabled on one side but not the other **How to check:** ``` Router# show interface gi0/0 | include MTU MTU 1500 bytes ``` **How to fix (option 1):** Match MTU on both routers: ``` Router(config-if)# ip mtu 1500 ``` **How to fix (option 2):** Tell OSPF to ignore MTU: ``` Router(config-if)# ip ospf mtu-ignore ``` **Learn more:** [OSPF MTU Mismatch Troubleshooting](https://www.pinglabz.com/ospf-mtu-mismatch-troubleshooting/) ### Neighbors Flapping (Up/Down) **Problem:** Adjacency forms, then drops, then forms again **Common causes:** - Hello/Dead timer mismatch - Intermittent link issues - Authentication misconfiguration - Duplicate Router ID **How to check timers:** ``` Router# show ip ospf interface gi0/0 | include Timer Timer intervals configured, Hello 10, Dead 40 ``` **How to check for duplicate Router ID:** ``` Router# show ip ospf %OSPF-4-DUP_RTRID1: Detected router with duplicate router ID 10.0.0.1 ``` **Learn more:** [OSPF Timers Explained](https://www.pinglabz.com/ospf-timers-hello-dead-intervals/), [Duplicate Router ID Troubleshooting](https://www.pinglabz.com/fix-duplicate-ospf-router-id/) ## Monitoring Neighbor State Changes ### See current neighbors ``` Router# show ip ospf neighbor ``` ### Watch neighbor state changes in real time ``` Router# debug ip ospf adj ``` **Example output:** ``` *Mar 17 12:34:56.789: OSPF-1 ADJ Gi0/0: Rcv hello from 192.168.1.2 area 0 *Mar 17 12:34:56.791: OSPF-1 ADJ Gi0/0: 2-Way communication to 192.168.1.2, state 2WAY *Mar 17 12:34:56.795: OSPF-1 ADJ Gi0/0: Nbr 192.168.1.2 has larger Router ID *Mar 17 12:34:56.796: OSPF-1 ADJ Gi0/0: Send DBD to 192.168.1.2 seq 0x123 opt 0x52 flag 0x7 len 32 *Mar 17 12:34:56.801: OSPF-1 ADJ Gi0/0: Rcv DBD from 192.168.1.2 seq 0x456 opt 0x52 flag 0x7 len 32 state EXSTART *Mar 17 12:34:56.802: OSPF-1 ADJ Gi0/0: NBR Negotiation Done. We are the SLAVE *Mar 17 12:34:56.850: OSPF-1 ADJ Gi0/0: Loading Done, state FULL ``` **Turn off debug:** ``` Router# undebug all ``` ⚠️ **Warning:** `debug` commands generate CPU-intensive output. Use carefully in production. ## Summary: Neighbor State Checklist Now you know: ✅ **8 OSPF neighbor states** and what each means ✅ **Normal state progression** from Down to Full ✅ **When 2-Way is normal** (DROthers on Ethernet) ✅ **When Full is expected** (point-to-point, DR/BDR adjacencies) ✅ **Common stuck states** and their causes (Init, ExStart, Exchange) ✅ **How to troubleshoot** state issues with `show` and `debug` commands **Next Step:** Understanding neighbor states is half the battle. The other half is understanding **why** DR and BDR exist. Read [**OSPF DR and BDR Explained**](https://www.pinglabz.com/ospf-dr-bdr-designated-router/) next. **Internal Links:** - ← [OSPF Key Terms and Concepts](https://www.pinglabz.com/ospf-key-terms-and-concepts/) - → [OSPF DR and BDR Explained](https://www.pinglabz.com/ospf-dr-bdr-designated-router/) - → [OSPF Neighbors Not Forming Troubleshooting](https://www.pinglabz.com/troubleshoot-ospf-neighbors-not-forming/) - → [OSPF MTU Mismatch Troubleshooting](https://www.pinglabz.com/ospf-mtu-mismatch-troubleshooting/) Take the OSPF reference with you The free OSPF field-reference PDF: neighbor states, LSA types, and the show commands you will actually run. Delivered by email, no card required. [Get the free PDF](https://www.pinglabz.com/ospf-cheatsheet/)