This tutorial explains how EIGRP speaking routers build and maintain a neighborship. Learn EIGRP neighborship requirements, conditions, and configuration values.
EIGRP routers share routing information only with neighbors. To build a neighborship, they use two configuration values. These values are AS number and K-values. Since the neighborship between two EIGRP routers depends on these configuration values, these configuration values are also known as EIGRP neighborship requirements.
Let's understand the meaning of these configuration values.
AS number
EIGRP routers use the concept of AS number to create a group of routers that can share routing information. Routers share routing information only within the group. For example, the following image shows two groups of routers. The AS number of the first group is 20 and the AS number of the second group is 30. Routers of the first group do not share routing information with the routers of the second group.
Key points: -
- An AS is a group of networks running under single administrative control.
- ASs are used to break a large network into smaller networks.
- An AS creates a boundary for routing protocol which allows us to control how far routing information should be propagated.
- An AS allows us to filter the routing information before sharing it with other AS systems.
- The AS feature enhances the security and scalability of the network.
- The AS concept was mainly developed for large networks.
- Routing protocols that were developed for small networks such as RIP do not understand the concept of AS systems.
- Cisco developed EIGRP for all types of networks. EIGRP understands and uses the concepts of AS.
- 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 the cost of routes. A composite metric includes multiple components in the calculation formula. K-values are the placeholder of these components. K-values allow an administrator to control the components used in the metric calculation formula. To become neighbors, two routers must use the same components in the metric calculation formula. If they use different components in the metric calculation formula, the result of the calculation will be different for the same path.
To learn what K-values are and how EIGRP uses them in the metric calculation formula, you can check the following tutorial.
Building an EIGRP neighborship
As mentioned earlier, EIGRP routers use two configuration values to form a neighborship. To share these values, they use Hello Packets. A Hello Packet is a special packet that contains the configuration that EIGRP routers use for various purposes.
When an EIGRP router joins a network, it sends Hello Packets out from all enabled interfaces. If an EIGRP router is connected to the other end of the enabled device, it receives the Hello Packet.
The second router checks the configuration values in the Hello Packet. If the AS number and K-values in the packet match the router's AS number and K-values, the router adds the first router into the Neighbor Table.
After adding the first router to the Neighbor Table, the second router responds with an update message. The update message contains all routing information the second router has.
The first router responds with an ACK message. This ACK message confirms the second router that the first router received the update message.
After sending the confirmation message, the first router sends an update message to the second router. This update message contains all routing information the first router has.
The second router responds with an ACK message. This ACK message confirms the first router that the second router received the update message.
At this point, both routers have converged. After this point, they do not share a full update. If they detect any change in the routing information, they share it through partial updates.
Maintaining an EIGRP neighborship
EIGRP uses Hello Packets to discover new EIGRP neighbors and maintain neighborship with active neighbors. EIGRP stores information about neighbors in the Neighbor Table.
When an EIGRP router forms a neighborship with another EIGRP router, it creates an entry for that router in the Neighbor Table. In simple words, the Neighbor Table contains information about all known and active neighbors.
When a router adds an entry for a neighbor to the Neighbor Table, it also associates a timer with the entry. This timer is called the hold timer. The router uses the hold timer to maintain active neighbors in the Neighbor Table. The hold time is the amount of time a router considers a neighbor is up without receiving a Hello Packet from the particular neighbor. The default hold time is 3 times of the default hello interval. The default hello interval is 5 seconds.
If the router does not receive a Hello Packet from a neighbor before the expiration of the hold time interval, the router declares the neighbor dead. After declaring the neighbor dead, the router ends the neighborship and deletes all entries associated with the neighbor.
Passive interface
EIGRP does not send Hello Packets out from a passive interface. To turn an interface passive, you can use the passive-interface command. Once you turned an interface passive, EIGRP will never send Hello Packets out from it. You can use this command to exclude an interface from all EIGRP operations.
Adjacency
EIGRP uses the term Adjacency to refer to the neighborship. In the log, it uses the term New Adjacency to refer to a new neighborship. The term New Adjacency indicates a new neighbor is found and the neighborship with the neighbor has been established.
Convergency
Convergency is a state that indicates 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.
To learn more about EIGRP message types, you can check the following tutorial.
That's all for this tutorial. In this tutorial, we discussed how EIGRP forms a neighborship and maintains the neighborship with existing neighbors.
