In networking, adjacency refers to the status of a connected neighbor where two routing devices have established a full operational relationship, allowing them to exchange routing information effectively. While neighbors are simply routers that sit on the same network segment and hear each other's presence, they do not engage in closer cooperation or route exchange until adjacency is formed. Adjacency signifies a higher level of trust and interaction. When devices achieve adjacency, based on specific adjacency parameters, they can exchange routes and maintain an updated view of the network topology.
Understanding Neighbors vs. Adjacency
It's crucial to distinguish between a simple neighbor relationship and a full adjacency. A router recognizes another device as a "neighbor" when it receives a "Hello" packet from it. However, this alone doesn't mean they can share routes. Adjacency implies a deeper, protocol-driven bond.
| Feature | Neighbor (Router) | Adjacent Neighbor (Router) |
|---|---|---|
| Relationship | Hear each other's presence (e.g., via Hello packets) | Full operational relationship; close cooperation and trust |
| Cooperation | Limited; basic discovery of existence | Extensive; synchronize databases and exchange routing tables |
| Route Exchange | No, routes are not exchanged at this stage | Yes, dynamic routes are exchanged and synchronized |
| Status | Basic connectivity confirmation | Advanced protocol-specific state (e.g., OSPF "Full" state) |
How Adjacency is Formed
Adjacency is a state achieved through a series of steps and agreement on specific parameters defined by dynamic routing protocols like OSPF (Open Shortest Path First) or EIGRP (Enhanced Interior Gateway Routing Protocol).
Key Adjacency Parameters and Conditions
For two devices to become adjacent, they must agree on several critical parameters:
- Matching Area ID (OSPF): For OSPF, connected interfaces must belong to the same OSPF area.
- Same Subnet/Network: Interfaces that are directly connected and intended to form adjacency must reside in the same IP subnet.
- Hello/Dead Timers: Protocols like OSPF use Hello packets to discover neighbors and maintain adjacency. The interval at which Hello packets are sent and the time a device waits for a Hello packet before declaring a neighbor "down" (dead timer) must typically match.
- Authentication: If routing protocol authentication is configured, both devices must use the same authentication type and credentials.
- Network Type (OSPF): OSPF operates differently on various network types (e.g., Broadcast, Point-to-Point, Non-Broadcast Multi-Access). These types must be compatible for adjacency to form.
- Unique Router IDs: Each router must have a unique Router ID within the routing domain.
- K-values (EIGRP): For EIGRP, the K-values, which are metrics used in the EIGRP composite metric calculation, must match between neighbors.
Adjacency Establishment Process (Example with OSPF)
OSPF, a link-state routing protocol, goes through several states to achieve full adjacency:
- Down: No Hello packets have been received from the neighbor.
- Init: A router has received a Hello packet from a neighbor, but its own Router ID is not yet seen in the neighbor's Hello packet.
- Two-Way: Bidirectional communication is established. Each router sees its own Router ID in the other's Hello packet. At this stage, on multi-access networks, a Designated Router (DR) and Backup Designated Router (BDR) are elected.
- ExStart/Exchange: Routers exchange Database Description (DBD) packets to describe their link-state databases.
- Loading: Routers request and exchange Link State Advertisements (LSAs) for any database entries that are missing or more recent.
- Full: Routers have fully synchronized their link-state databases and are considered fully adjacent. At this point, they can exchange routes and begin calculating the best paths.
Importance of Adjacency in Networking
Adjacency is fundamental for the operation of modern, dynamic networks:
- Dynamic Route Exchange: It enables routers to automatically learn and share network reachability information, eliminating the need for manual static route configuration and adapting to changes.
- Network Topology Awareness: Allows routers to build and maintain an accurate and up-to-date map of the entire network, which is essential for making optimal forwarding decisions.
- Fault Tolerance and Redundancy: Adjacent routers can quickly detect network changes or failures and update their routing tables, ensuring continuous connectivity and fast convergence.
- Scalability: Facilitates the growth of networks by automating routing updates, allowing new routers or network segments to be integrated seamlessly.
Troubleshooting Adjacency Issues
Common problems that prevent adjacency from forming usually stem from misconfigurations of the parameters mentioned above.
- Mismatched Parameters: Always verify Hello/Dead timers, subnet masks, OSPF area IDs, EIGRP K-values, and authentication settings between the two devices.
- Access List Filtering: Ensure that any configured firewalls or Access Control Lists (ACLs) are not blocking routing protocol traffic (e.g., OSPF uses IP protocol 89; EIGRP uses IP protocol 88).
- Interface Status: Confirm that the physical and logical interfaces on both routers are up and operational.
- Network Type (OSPF): An incorrect OSPF network type configuration can prevent adjacency, especially on non-broadcast multi-access (NBMA) networks.
- Unicast vs. Multicast: Ensure routers can send and receive the necessary protocol packets. For example, OSPF uses multicast addresses like 224.0.0.5 and 224.0.0.6.
