EIGRP saves routes that are learned in specific ways. Routes are given a particular status and can be tagged to provide additional useful information.

EIGRP maintains three tables:

• Neighbor table
• Topology table
• Routing table

The neighbor table is the most important table in EIGRP. Each EIGRP router maintains a neighbor table that lists adjacent routers. This table is comparable to the adjacency database used by OSPF. There is a neighbor table for each protocol that EIGRP supports.

When newly discovered neighbors are learned, the address and interface of the neighbor is recorded. This information is stored in the neighbor data structure. When a neighbor sends a hello packet, it advertises a hold time. The hold time is the amount of time a router treats a neighbor as reachable and operational. In other words, if a hello packet is not heard within the hold time, then the hold time expires. When the hold time expires, the Diffusing Update Algorithm (DUAL), which is the EIGRP distance vector algorithm, is informed of the topology change and must recalculate the new topology.

The topology table is made up of all the EIGRP routing tables in the autonomous system. DUAL takes the information supplied in the neighbor table and the topology table and calculates the lowest cost routes to each destination. By tracking this information, EIGRP routers can identify and switch to alternate routes quickly. The information that the router learns from the DUAL is used to determine the successor route, which is the term used to identify the primary or best route. A copy is also placed in the topology table.

Every EIGRP router maintains a topology table for each configured network protocol. All learned routes to a destination are maintained in the topology table.

The topology table includes the following fields:

• Feasible distance (FD is 2195456) 200.10.10.10 – The feasible distance (FD) is the lowest calculated metric to each destination. For example, the feasible distance to 32.0.0.0 is 90 as indicated by FD is equal 90.
• Route source (via 200.10.10.10) – The source of the route is the identification number of the router that originally advertised that route. This field is populated only for routes learned externally from the EIGRP network. Route tagging can be particularly useful with policy-based routing. For example, the route source to 32.0.0.0 is 200.10.10.10 via 200.10.10.10.
• Reported distance (FD/RD) – The reported distance (RD) of the path is the distance reported by an adjacent neighbor to a specific destination. For example, the reported distance to 32.0.0.0 is 2195456 as indicated by (90/2195456).
• Interface information – The interface through which the destination is reachable
• Route status – Routes are identified as being either passive (P), which means that the route is stable and ready for use, or active (A), which means that the route is in the process of being recomputed by DUAL.

The EIGRP routing table holds the best routes to a destination. This information is retrieved from the topology table. Each EIGRP router maintains a routing table for each network protocol.

A successor is a route selected as the primary route to use to reach a destination. DUAL identifies this route from the information contained in the neighbor and topology tables and places it in the routing table. There can be up to four successor routes for any particular route. These can be of equal or unequal cost and are identified as the best loop-free paths to a given destination. A copy of the successor routes is also placed in the topology table.

A feasible successor (FS) is a backup route. These routes are identified at the same time the successors are identified, but they are only kept in the topology table. Multiple feasible successors for a destination can be retained in the topology table although it is not mandatory.

A router views its feasible successors as neighbors downstream, or closer to the destination than it is. Feasible successor cost is computed by the advertised cost of the neighbor router to the destination. If a successor route goes down, the router will look for an identified feasible successor. This route will be promoted to successor status. A feasible successor must have a lower advertised cost than the existing successor cost to the destination. If a feasible successor is not identified from the existing information, the router places an Active status on a route and sends out query packets to all neighbors in order to recompute the current topology. The router can identify any new successor or feasible successor routes from the new data that is received from the reply packets that answer the query requests. The router will then place a Passive status on the route.

The topology table can record additional information about each route. EIGRP classifies routes as either internal or external. EIGRP adds a route tag to each route to identify this classification. Internal routes originate from within the EIGRP autonomous system (AS).

External routes originate outside the EIGRP AS. Routes learned or redistributed from other routing protocols, such as Routing Information Protocol (RIP), OSPF, and IGRP, are external. Static routes originating outside the EIGRP AS are external. The tag can be configured to a number between 0-255 to customize the tag.