EIGRP Tutorial – Basic concept explained

EIGRP is a proprietary routing protocol developed by Cisco. It dynamically discovers, adds, and manages routes in the routing table. It uses complex terminology to describe its components and functions. This tutorial describes its key terms and their meanings.

EIGRP routers exchange routing information only with neighbors. If two EIGRP routers are not neighbors, they will not exchange routing information even if they have a direct connection. To become neighbors, they match several parameters. They use hello packets to exchange these parameters.

When you start an EIGRP router, it sends hello messages to identify potential neighbors. A hello message includes all the necessary parameters that EIGRP running routers need to become neighbors. If a router receives a hello message on any interface, it compares the hello packet’s parameters with the locally configured values. If both match, it adds the sender router to its neighbor table and replies to the hello message. The sender router receives the reply message and compares the parameters. If both match, it adds the router to its neighbor table. After becoming neighbors, they exchange routing information.

Through this entire process, EIGRP uses many terms to define various functions and their components. The following section explains them.

EIGRP Neighbor

An EIGRP neighbor is another router running EIGRP on the same subnet that can exchange routing information. Two EIGRP running routers become neighbors only if the following conditions are met.

• If authentication is configured, both routers must use the same type of authentication and the same authentication key.
• The AS (Autonomous System) number must match on both routers.
• The interfaces that exchange EIGRP Hello messages must belong to the same IP subnet.
• The K values must match on both routers.

AS (Autonomous System) number

AS numbers group routers. Routers share routing information within the group. An AS is a single network or a group of networks in which all routers share routing information. If two routers belong to two different AS, they do not share routing information.

Hello packets/messages

EIGRP uses hello packets to discover potential EIGRP neighbors and to maintain them. EIGRP uses the multicast address 224.0.0.10 as the destination address in hello packets.

Hello timer

By default, EIGRP generates hello packets every 5 seconds, a period known as the hello timer. This interval can be adjusted as needed.

Hold timer

The hold timer specifies the duration a router instructs others to wait before considering it unreachable. Once a neighbor is declared unreachable, EIGRP removes it from the neighbor table and recalculates all dependent routes. The default hold timer is three times the hello timer, typically 15 seconds. This value is configurable.

EIGRP metric

EIGRP uses a composite metric to calculate the best route. The composite metric uses the five components: bandwidth, delay, load, reliability, and MTU. You can enable and disable each component. EIGRP includes only enabled components in the metric. By default, it uses only bandwidth and delay in the metric. If you want to add another component, you must enable it.

Update message

An EIGRP-running router uses update messages to transmit its topology information to another EIGRP-running router.

Query message

An EIGRP-running router uses a query message to request validation of routing information from a neighboring router. The neighboring router sends a reply message in response to a query message.

Neighbor table

The neighbor table stores a list of all neighbors. It maintains a separate table for each routed protocol.

Topology table

The topology table contains all learned destinations and paths, with a distinct table maintained for each routed protocol.

Successor

A successor route represents the optimal path to a destination in the topology table. If multiple paths exist, it selects the one with the lowest metric as the successor.

Feasible successor

A feasible successor is the most suitable backup path to a destination in the topology table. When multiple paths are available, the path with the lower metric is chosen as the successor, and the next-best path becomes the feasible successor.

Routing table

EIGRP adds all successor paths to the routing table. Routers use routing tables to make forwarding decisions.

Advertised distance

This term refers to the metric of a subnet as reported or advertised by a neighbor. The neighbor router uses this metric in its routing table, while the local router stores it alongside the path in its topology table.

Feasible Distance (FD)

This term refers to the subnet metric calculated by the local router. The router computes its own metric and adds it to the advertised distance to determine the feasible distance for the subnet.

Reliable Transport Protocol (RTP)

EIGRP uses the Reliable Transport Protocol (RTP), a Cisco-proprietary protocol, to facilitate communication between EIGRP routers. RTP supports both unicast and multicast transmissions to ensure reliable communication.

Conclusion

EIGRP is a robust and efficient routing protocol developed by Cisco. Its advanced metrics, dynamic neighbor discovery, and reliable communication methods simplify route management and enable rapid convergence in dynamic network environments. Understanding its terminology and operations is essential for effective configuration and troubleshooting of Cisco networks.