EIGRP Tutorial – Basic concept explained

This tutorial explains EIGRP terminology and fundamentals. Learn the basic concepts and operations of the EIGRP routing protocol.

EIGRP is a Cisco propriety routing protocol. It dynamically discovers, adds, and manages routes on the router. It works in three steps. These steps are the following.


An EIGRP router joins the network and sends Hello messages to discover potential neighboring EIGRP routers. EIGRP neighboring routers reply to Hello messages. Hello messages and reply messages contain the required parameters to become a neighbor. The EIGRP router and EIGRP neighboring routers check parameters to determine which routers should become neighbors. Neighbors that pass all parameters build a neighbor relationship.


When building a neighbor relationship, EIGRP routers exchange full topology updates. After this, they only share partial updates as needed based on changes to the network topology. EIGRP routers stores topology information into the EIGRP topology table.


An EIGRP router chooses the lowest-metric route to reach each subnet from the EIGRP topology table and places the route with the metric into the routing table. The router uses the routing table to forward packets.

EIGRP Terminology

EIGRP is a complex protocol. It uses several terms to refer to its components and functions. Let’s discuss these terms and their meanings.

EIGRP Neighbor

From an EIGRP router's perspective, an EIGRP neighbor is another EIGRP running router that is connected to the same subnet and ready to share routing information with the first router.

Two EIGRP running routers become neighbors only if the following information matches.

  • If authentication is configured, both routers must use the same type of authentication and the same authentication key.
  • The AS (Autonomous System) number must be the same 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

Administrators use AS numbers to create groups of 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.

AS numbers

Hello packets/messages

EIGRP uses Hello packets to discover potential EIGRP neighbors and maintain the EIGRP neighbors. EIGRP uses the multicast address for the destination in the hello packets.

Hello timer

By default, EIGRP generates hello packets every 5 seconds. This time interval is known as the hello timer. If required, you can adjust this timer.

Hold timer

The hold timer is the amount of time a router tells others to waits before they can declare it dead. Once a neighbor is declared dead, EIGRP removes it from the neighbor table and recalculates all routes that depend on it. The default value of this timer is three times (5 * 3 = 15 seconds) of the hello timer. You can also adjust this value.

EIGRP metric

EIGRP uses a composite metric to calculate the best route. A composite metric is a metric that uses more than one component and uses the math equation to calculate the result. EIGRP uses five components in the composite metric. These components are bandwidth, delay, load, reliability, and MTU.


K-values are the placeholder for components used in the metric calculation formula. You can use K-values to control the components of the EIGRP metric calculation formula. By default, EIGRP uses only bandwidth and delay in the formula. If you want to add another component to the formula, you have to enable its K-value. In simple words, K-values are used to enable or disable the different metric components used in the metric calculation formula.

Update message

A router uses an update message to send its topology to another router. The router uses this message when it builds a neighbor relationship with another router.

Query message

A router uses a query message to ask a neighboring router to validate routing information.

Reply message

A router uses a reply message to respond to a query message.

Neighbor table

EIGRP uses the neighbor table to store a list of the EIGPR neighbors. EIGRP uses a separate neighbor table for each routed protocol.

Topology table

EIGRP uses the topology table to store a list of all destinations and paths it learned. It uses a separate topology table for each routed protocol.


A successor route is the best path to reach a destination within the topology table. If there is only one path to reach a destination, EIGRP selects the available path as successor. If there is more than one path to reach a destination, EIGRP selects the path that has the lowest metric as a successor.

Feasible successor

A feasible successor is the best backup path to reach a destination within the topology table. If there are two paths to reach a destination, the path with the lower metric will be selected as a successor and the path with the higher metric will be selected as a feasible successor.

feasible successor

Routing table

EIGRP places all successor paths into the routing table. A router uses the routing table to make forwarding decisions.

Advertised distance (AD) / Reported distance (RD)

This is the metric of a subnet, as reported or advertised by a neighbor. The neighbor router uses the same metric in the routing table and the local router keeps this metric with the path in the topology table.

Feasible Distance (FD)

This is the metric of a subnet as calculated by the local router. The local router first calculates its metric then adds the result to the advertised distance to calculate the feasible distance of a subnet.

Reliable Transport Protocol (RTP)

EIGRP uses RTP to communicate with other EIGRP running routers. RTP is a Cisco proprietary protocol. Cisco developed this protocol for reliable communication between EIGRP routers. This protocol uses both unicast and multicast to provide reliable communication.

That's all for this tutorial. In this tutorial, we discussed EIGRP terminology and learned the basic concept and operation of EIGRP.

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