Neighbor adjacencies are fundamental relationships formed between routing devices in a network to exchange routing information and maintain network connectivity. In OSPF (Open Shortest Path First) and other dynamic routing protocols, routers must establish these adjacencies before they can share ro…Neighbor adjacencies are fundamental relationships formed between routing devices in a network to exchange routing information and maintain network connectivity. In OSPF (Open Shortest Path First) and other dynamic routing protocols, routers must establish these adjacencies before they can share routing updates.
The adjacency formation process begins when routers discover each other through Hello packets. These packets are sent periodically on enabled interfaces and contain information such as Router ID, Hello/Dead intervals, Area ID, and authentication data. For an adjacency to form, certain parameters must match between neighboring routers.
In OSPF, the adjacency process follows specific states: Down, Init, Two-Way, ExStart, Exchange, Loading, and Full. During the Down state, no Hello packets have been received. The Init state occurs when a router receives a Hello but hasn't seen its own Router ID in the neighbor's Hello. Two-Way state indicates bidirectional communication has been established. From ExStart through Full, routers negotiate master/slave relationships, exchange Database Description packets, request missing LSAs, and ultimately synchronize their link-state databases.
On multi-access networks like Ethernet, OSPF elects a Designated Router (DR) and Backup Designated Router (BDR) to reduce the number of adjacencies required. Non-DR/BDR routers only form full adjacencies with the DR and BDR, remaining in Two-Way state with other neighbors.
EIGRP also uses neighbor adjacencies but employs a simpler process. Routers exchange Hello packets and form adjacencies when parameters match. EIGRP neighbors share routing information through Update packets and maintain relationships through periodic Hellos.
Troubleshooting adjacency issues involves verifying matching timers, authentication credentials, subnet configurations, and area assignments. Commands like 'show ip ospf neighbor' or 'show ip eigrp neighbors' display current adjacency states and help identify problems in neighbor relationships.
OSPF Neighbor Adjacencies - Complete Study Guide
Why OSPF Neighbor Adjacencies Are Important
OSPF neighbor adjacencies form the foundation of OSPF routing. Before routers can exchange routing information, they must first establish a neighbor relationship and then form an adjacency. Understanding this process is critical for troubleshooting OSPF issues and is heavily tested on the CCNA exam.
What Are OSPF Neighbor Adjacencies?
An OSPF neighbor relationship exists when two routers discover each other and agree to exchange Hello packets. An adjacency is a more advanced relationship where routers actually exchange link-state advertisements (LSAs) and synchronize their databases.
OSPF Neighbor States
OSPF routers progress through several states when forming adjacencies:
1. Down State: No Hello packets received from the neighbor.
2. Init State: Hello packet received, but the local router ID is not listed in the neighbor's Hello.
3. 2-Way State: Bidirectional communication established. Both routers see each other's Router ID in Hello packets. DR/BDR election occurs here on multi-access networks.
4. ExStart State: Master/Slave relationship established for database exchange.
7. Full State: Databases are synchronized. Adjacency is fully established.
Requirements for OSPF Neighbor Formation
For two routers to become OSPF neighbors, these parameters must match:
- Hello and Dead Intervals: Default Hello is 10 seconds on broadcast networks, 30 seconds on non-broadcast. Dead interval is 4x the Hello interval.
- Area ID: Both interfaces must be in the same OSPF area.
- Subnet Mask: Must match on the connecting interfaces.
- Authentication: Type and credentials must match if configured.
- Stub Area Flags: Must agree on stub area configuration.
- MTU: Must match for adjacency to reach Full state (can be disabled with ip ospf mtu-ignore).
DR and BDR Elections
On multi-access networks (Ethernet), OSPF elects a Designated Router (DR) and Backup Designated Router (BDR):
- Highest OSPF priority wins (default is 1, range 0-255) - Priority of 0 means the router will never become DR/BDR - If priority is tied, highest Router ID wins - Elections are non-preemptive
Useful Verification Commands
show ip ospf neighbor - Displays neighbor table with states show ip ospf interface - Shows OSPF interface details including timers show ip ospf database - Displays the LSDB
Exam Tips: Answering Questions on Neighbor Adjacencies
1. Memorize the neighbor states in order: Down, Init, 2-Way, ExStart, Exchange, Loading, Full. Know what happens at each state.
2. Remember the matching requirements: When troubleshooting neighbor issues, always check Hello/Dead timers, Area ID, subnet mask, and authentication first.
3. Understand 2-Way vs Full: On multi-access networks, DROther routers remain in 2-Way state with each other. Only the relationship with DR and BDR reaches Full state.
4. Know DR/BDR election rules: Priority first, then Router ID. Remember elections are non-preemptive.
5. Stuck in ExStart/Exchange: This typically indicates an MTU mismatch between neighbors.
6. Stuck in Init: One-way communication issue - check ACLs blocking OSPF, or interface issues.
7. Read questions carefully: Pay attention to whether the question asks about neighbor requirements vs adjacency requirements.
8. Practice command output interpretation: Be able to identify the neighbor state and DR/BDR roles from show ip ospf neighbor output.