When an OSPF router is first connected to a network, it follows a link-state routing process to achieve convergence. This involves building adjacencies, exchanging information, and progressing through various neighbor states. We’ll start with the overall OSPF process before diving into the states.
OSPF Process Overview
Step 1: Create Adjacencies with Neighbors
An OSPF-configured router forms adjacencies with its neighbors before sharing routing information. It sends Hello packets out all OSPF-enabled interfaces to discover connected OSPF neighbors. If a neighbor is found, it attempts to establish an adjacency.
- Hello Packet Details: Includes Router ID, area ID, authentication, Hello/Dead intervals, and DR/BDR info.
- For CCNA/CCNP: Mismatched Hello/Dead timers prevent adjacency—use show ip ospf interface to verify.
Step 2: Exchange Routing Information Using LSAs
Once adjacencies are established, routers exchange Link-State Advertisements (LSAs) containing the state and cost of each directly connected link. Routers flood LSAs to adjacent neighbors, which then flood them further until all routers in the area have identical LSAs.
- Key LSA Types for CCNA/CCNP:
- Type 1: Router LSA (describes router links).
- Type 2: Network LSA (from DR in broadcast networks).
- Flooding Process: Ensures a synchronized Link-State Database (LSDB).
Step 3: Build the Topology Table (LSDB)
After receiving LSAs, OSPF routers build the Link-State Database (LSDB), which holds all topology information. All routers in the same area must have an identical LSDB for convergence.
- Example: In a single-area OSPF, the LSDB includes all routers’ links and costs.
- CCNP Tip: In multi-area OSPF, ABRs summarize LSAs to reduce LSDB size.
Step 4: Execute the SPF Algorithm
Routers run the Shortest Path First (SPF) algorithm (Dijkstra’s) on the LSDB to build an SPF tree, with the local router as the root.
Step 5: Calculate and Install Best Routes
From the SPF tree, the router selects lowest-cost paths and inserts them into the routing table (e.g., via ip route entries).
- For CCNA: Cost = 10^8 / bandwidth (default).
- Troubleshooting: Use show ip ospf database to verify LSDB before SPF runs.
Step 6: Reach Convergence
Convergence occurs when the routing table is populated with best paths, and all routers have consistent views. OSPF achieves this by progressing through neighbor states (detailed below).
OSPF Neighbor States Progression
OSPF routers progress through these states to form adjacencies and achieve convergence. The attached figure illustrates this flow.
This visual represents the sequence of OSPF neighbor states (Down, Attempt, Init, 2-Way, ExStart, Exchange, Loading, Full) that a router progresses through to establish adjacencies, exchange LSAs, and reach full convergence. Arrows indicate key transitions, such as sending Hello packets or negotiating master/slave relationships, making it ideal for understanding OSPF operations in CCNA and CCNP studies.
1. Down State
No Hello packets exchanged. Router is powered off or interface down.
- Transition: Powers up and sends Hello → Attempt/Init.
2. Attempt State
Used in NBMA networks for manual neighbors. Router tries to contact but no response yet.
- CCNA Tip: Rare in modern Ethernet; focus on broadcast networks.
3. Init State
Received Hello from neighbor, but bidirectional communication not confirmed.
- Transition: Sees own Router ID in neighbor’s Hello → 2-Way.
4. 2-Way State
Bidirectional communication established. DR/BDR elected on Ethernet/broadcast links.
- CCNP Tip: Stays in 2-Way with non-DR/BDR on multi-access networks.
5. ExStart State
Negotiates master/slave (higher Router ID wins) for DBD exchange.
- Transition: Master starts DBD → Exchange.
6. Exchange State
Exchanges Database Description (DBD) packets summarizing LSAs.
- Transition: If more info needed → Loading; else → Full.
7. Loading State
Requests/updates LSAs using LSR/LSU packets.
- Transition: LSDB synchronized → Full.
8. Full State
Adjacency complete; LSDB identical. Convergence achieved; periodic LSAs exchanged.
- Verification: Use show ip ospf neighbor to check states.
FAQs
What are the main OSPF neighbor states?
OSPF neighbor states include Down (no Hellos, powered off), Attempt (NBMA manual contact), Init (Hello received, no bidirectional), 2-Way (bidirectional, DR/BDR elected), ExStart (master/slave negotiation), Exchange (DBD summaries exchanged), Loading (LSR/LSU for details), and Full (LSDB synced, converged). These progress to form adjacencies and achieve routing stability.
How does OSPF exchange routing information?
After adjacencies form via Hello packets, routers exchange LSAs (e.g., Type 1 Router, Type 2 Network) containing link states and costs. Flooding ensures all area routers get identical LSAs, building the LSDB for SPF calculations. Mismatches in timers or authentication prevent this step.
What is the role of the SPF algorithm in OSPF?
The SPF (Dijkstra’s) algorithm runs on the LSDB to create an SPF tree with the local router as root, calculating lowest-cost paths. These are installed in the routing table for decisions. Default cost is 10^8/bandwidth; verify LSDB with ‘show ip ospf database’ before SPF.
Why might an OSPF neighbor stay in 2-Way state?
In broadcast networks, non-DR/BDR neighbors remain in 2-Way for efficiency, while only DR/BDR reach Full with all. Check with ‘show ip ospf neighbor’. Issues like priority 0 or mismatches prevent election. Focus on Ethernet for CCNA; multi-access for CCNP.
How do you troubleshoot OSPF convergence issues?
Use commands like ‘show ip ospf interface’ for timers, ‘show ip ospf neighbor’ for states, ‘show ip ospf database’ for LSDB consistency. Common problems: area ID mismatches, authentication failures, MTU issues. In multi-area, check ABR summarization.
