EIGRP Neighborship Requirements and Conditions

EIGRP routers share routing information only with their neighbors. To build a neighborship, they compare and match the AS (Autonomous System) number and K-values. If these values match, they become neighbors. If not, they ignore each other. Since neighborship depends on these, these values are known as the EIGRP neighborship requirements.

AS number

EIGRP routers use AS numbers to form groups of routers that can exchange routing information. Routers share routing information only within their respective groups. For example, if one group has an AS number of 20 and another has an AS number of 30, routers in the first group do not share routing information with those in the second group.

Key points:-

• An AS is a group of networks running under a single administrative control.
• Administrators use AS to break a large network into smaller networks.
• An AS defines a boundary for a routing protocol, enabling control over the scope of routing information propagation.
• An AS allows filtering of routing information before it is shared with other AS systems.
• It enhances network security and scalability.
• It was mainly developed for large networks.
• Routing protocols developed for small networks, such as RIP, do not support AS systems.
• Cisco developed EIGRP for all types of networks. EIGRP understands and uses AS concepts.
• EIGRP routers that belong to different ASs don't become neighbors; therefore, they don't share any routing information.

K-values

EIGRP routers use a composite metric to calculate route costs, which incorporates multiple components. K-values serve as placeholders for these components, allowing administrators to determine which components are used in the metric calculation. Two routers become neighbors only if they use identical components in their metric calculation formula.

Example

Two EIGRP routers, R1 and R2, are connected in a network. When you start them, they have no information about each other. R1 transmits hello packets from all enabled interfaces. R2 is present on the connected segment. It receives hello packets. A hello packet includes all configuration values needed to build the neighborship.

R2 compares the configuration values it received in the hello packet with its own configuration values. If both match, it adds R1 to its neighbor table. After adding R1 to its neighbor table, R2 sends an update message containing all its routing information to R1.

R1 replies with an ACK message to confirm receipt of the update message.

After sending the confirmation, R1 transmits an update message to R2 that includes all its routing information.

R2 replies with an ACK, confirming receipt of the update.

At this stage, both routers have converged. Subsequently, they do not exchange full updates. Any changes in routing information are communicated through partial updates.

Maintaining an EIGRP neighborship

EIGRP uses hello packets to discover new neighbors and maintain neighborships with active routers. When an EIGRP router establishes a neighborship, it creates an entry for the neighbor in the neighbor table. The neighbor table contains information about all known and active neighbors. When a router adds a neighbor to its neighbor table, it associates a hold timer with the entry. The hold timer determines how long a neighbor is considered active without receiving a hello packet. By default, the hold time is three times the default hello interval, which is five seconds.

If a router does not receive a hello packet from a neighbor before the hold timer expires, it declares the neighbor unreachable, terminates the neighborship, and removes all associated entries.

Passive interface

EIGRP does not send hello packets from a passive interface. The passive-interface command designates an interface as passive, thereby excluding it from all EIGRP operations.

Adjacency

EIGRP uses the term Adjacency to denote neighborship. In system logs, New Adjacency indicates that a new neighbor has been discovered and a neighborship has been established.

Convergency

Convergence indicates that two EIGRP routers have learned all routing information from each other.

Full update

A full update contains all routing information the source router has and wants to share with another router. To send a full update, EIGRP uses an update message.

Partial update

A partial update contains only the recent change in topology or a link. EIGRP uses Query and Reply messages to learn and share partial updates.

Conclusion

EIGRP neighborship is fundamental to the efficient and reliable exchange of routing information in a network. By requiring matching AS numbers and K-values, EIGRP ensures that only compatible routers form neighbor relationships, which enhances network stability and security. The process of forming and maintaining these neighborships through Hello, Update, and ACK messages enables routers to rapidly share and synchronize routing data. Proper understanding and configuration of EIGRP neighborship parameters are essential for robust network design and smooth operation.