EIGRP concepts and terminology

Hello everyone, I'm continuing from last topic where we covered overview now it turn for concepts and terminology.

Now let’s have look at concepts and terminology, EIGRP routers keep route and topology info always available in RAM, so that EIGRP routers can  react quickly to situation of changes. Just Like Routing protocol OSPF, EIGRP saves this info in several different tables and databases.

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

Routing Protocol EIGRP maintains three types tables:

• Neighbour table
• Topology table
• Routing table

Neighbour Table: The neighbour table is the most important table in EIGRP. Each EIGRP router maintains a neighbour table that lists the routers which are adjacent . Neighbour table is commensurte to the adjacency database used by routing protocol OSPF. There is a neighbour table for each protocol that routing protocol EIGRP Assist.

EIGRP Neighbour Table

So, As the newly discovered neighbours are learned, thier respective address and interface of the neighbour is recorded.Which is stored in the neighbour data structure. Firstly a neighbour sends a hello packet, that advertises a hold time, the duration of time a router treats a neighbour as reachable and operational. In other words, let’s say if a hello packet is not heard within the hold time, then the hold time expires. Rsulting in informing Diffusing Update Algorithm (DUAL) of the topology change and must recalculate the new topology. EIGRP uses DUAL as distance vector algorithm.

All the EIGRP routing tables in the autonomous system are part of Topology table.it’s The responsibility of DUAL that is Diffusing Update Algorithm to takes the information from the neighbour table and the topology table and then calculates the lowest cost routes to each destination. 

EIGRP Routing Table

You might have a question in your mind, how EIGRP identify and switch to alternate routes so quickly?. So Secret is information(we talked about it previous paragraph), 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 of it also placed in the topology table.

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

Now we will have a look the type of field topology table include, following are the 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. Interstingly, 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) –it is the distance reported by an adjacent neighbour 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.

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

 Successor: 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 neighbour and topology tables and places it in the routing table. There can be maximum of 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.

There is also a backup route which is called feasible successor. These routes are identified at the same time the successors are identified, but they are only kept in the topology table. A destination can have Multiple feasible successors in the topology table although it is not obligatory.

Alternative routes using EIGRP Successors and Feasible Successors

EIGRP Successors and Feasible Successors

A router views its feasible successors as neighbours downstream, or nearer to the destination than it is. Feasible successor cost is concluded by the advertised cost of the neighbour router to the destination. If a successor route fails, the router will look for an identified or known feasible successor. This route will be forward or promoted  to successor status. A feasible successor must have a lower advertised cost than the existing successor cost to the destination. however If a feasible successor is not identified from the existing information, Then the router places an Active status on a route and sends out query packets to all neighbours so as to recompute the current topology. The router can identify any new successor or feasible successor routes after receiving new data from the reply packets that answer the query requests. After that The router will then place a Passive status on the route.

Some Additional information about each route is also recored in topology table. 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.  

Viewing EIGRP Tag Information