EIGRP stands for Enhanced Interior Gateway Routing Protocol.  

It is a Cisco-proprietary routing protocol for TCP/IP. It is based on IGRP routing protocol. It has several enhancements to scale the enterprise size network.
Features and characteristics of EIGRP
 It is a Cisco Proprietary routing protocol.  It is based on IGRP Routing protocol.  It is an enhanced version of IGRP (Interior Gateway Routing Protocol) protocol.  In comparison of IGRP it provides faster convergence times, superior handling of routing loops and improved scalability.  It was released in 1994.  It is a hybrid routing protocol.  It has characteristics of both distance vector and link state protocols.  It uses DUAL (Diffusing Update Algorithm) algorithm to select the best path.  It uses RTP (Reliable Transport Protocol) to communicate with neighbors.  It uses multicast for routing updates.  It supports IP [Both IPv4 and IPV6], Apple Talk and IPX routed protocols.  It includes subnet mask information in routing updates.  It supports route summarization and discontiguous networks.  It supports VLSM/CIDR.  It supports load balancing across the six routes for a single destination.  It supports trigger updates.
Since EIGRP is hybrid protocol, it has advantages of both link state and distance vector protocol. It uses composite metric calculation formula to select the best route for destination. It sends partial or full update only when something is change in network. It maintains three tables for ultra-fast convergence.
1. Neighbor Table 2. Topology Table 3. Routing Table
Neighbor Table
EIGRP shares routing information only with neighbors. To know who the neighbors are, it uses neighbor table. When a new neighbor is discovered, EIGRP would add its address and interface on which neighbor is connected in neighbor table. EIGRP uses separate neighbor table for each routed protocol.
Topology Table
EIGRP uses this table to store all routes which it learned from neighbors. It contains a list of all destinations and routes advertised by neighboring routers. EIGRP selects single best route for each destination from this list. That route goes in routing table. Remaining routes are marked as backup routes. EIGRP refers selected route as Successor and backup route as Feasible Successor. EIGRP uses separate topology table for each routed protocol.
Routing Table
EIGRP stores single best (Successor) route for each destination in this table. Router uses this table to forward the packet. There is a separate routing table for each routed protocol.
Protocol Dependent Modules
PDMs are the special feature of EIGPR. Through these modules EIGRP supports multiple network layer protocols. It maintains separate tables for separate routed (Network Layer) protocols. For example if you are using both (IPv4 and IPv6) versions of IP protocol, it will maintain separate IPv4/EIGRP and IPv6/EIGRP tables.
DUAL
EIGRP uses DUAL (Diffusing Update Algorithm) to provide the fastest route convergence among all protocols. Route convergence includes:-
 Selecting best route from all available routes  Supporting VLSMs
 Dynamically recovering from route failure  Finding an alternative route if primary route goes down
DUAL uses topology table along with RTP to accomplish above tasks in minimal time. As we know EIGRP maintain a copy of all routes including neighbors in topology table, so it would be the first place to look for an alternative route in a route failure situation. If EIGRP does not find an alternative here, it will ask neighbors for help. If neighbors have any updates about asked route, they will reply back with that information. This strong mechanism allows DUAL to find and maintain the best routes for destination speedily.
Autonomous System
EIGRP shares routing information only with neighbors. In order to become a neighbor AS number must be matched. AS create a logical boundary for route information. By default router will not propagate route information outside the AS. For example a router which belongs to AS number 10 will not share routing information with the router that belongs to AS number 20 or any other AS numbers except AS number 10. For easy administration a large network may have multiple ASes.
Not all routing protocols understand the concept of AS. Luckily EIGRP not only understand the concept of AS but also supports multiple ASes. We can easily configure multiple AS instance with EIGRP to divide a large network in smaller segments. By default EIGRP routers will not share routing information between different AS.
Redistribution is used to exchange the route information between different ASes. When a route is learned through the redistribution, it has higher AD value than its original source. For example EIGRP has two AD values 90 for interior EIGRP and 170 for exterior EIGRP. Exterior EIGRP means EIGRP instance which has different AS number.
Enhanced Interior Gateway Routing Protocol (EIGRP) Configuration
Enhanced Interior Gateway Routing Protocol (EIGRP) can be configured in a router using the following IOS commands.
Router(config)# router eigrp ASN Router(config-router)# network Network_ID
ASN in the above IOS command stands for Autonomous System Number.
Enhanced Interior Gateway Routing Protocol (EIGRP) - Lab Practice
The following diagram shows our lab setup. We have three routers, three switches and three hosts connected as below. The host names, IP addresses and the interfaces of the routers are shown in diagram. The IP addresses of the hosts are also shown in the diagram.
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Hostname and IP address configuration in Router01
Connect to Router01 console and use the following IOS commands to configure host name as Router01.
Router>enable
Router#configure terminal Enter configuration commands, one per line.  End with CNTL/Z. Router(config)#hostname Router01 Router01(config)#
Use the following IOS commands to open the fast ethernet interface Fa0/0 configuration mode on Router01 and configure IP address as 172.16.0.1/16.
Router01>enable  Router01#configure terminal Enter configuration commands, one per line.  End with CNTL/Z. Router01(config)#interface fa0/0 Router01(config-if)#ip address 172.16.0.1 255.255.0.0 Router01(config-if)#no shutdown
Use the following IOS commands to open the serial interface S0/0 configuration mode on Router01 and configure IP address as 172.17.0.1/16. You have to set a clock rate also using the "clock rate" command on S0/0 interface, since this is the DCE side.
Router01>enable Router01#configure terminal Enter configuration commands, one per line.  End with CNTL/Z. Router01(config)#interface s0/0 Router01(config-if)#clock rate 64000 Router01(config-if)#ip address 172.17.0.1 255.255.0.0 Router01(config-if)#no shutdown
Do remember to run the "copy running-config startup-config" command from enable mode, if you want to save the changes you have made in the router.
Hostname and IP address configuration in Router02
Connect to Router02 console and use the following IOS commands to configure host name as Router02.
Router>enable Router#configure terminal Enter configuration commands, one per line.  End with CNTL/Z. Router(config)#hostname Router02 Router02(config)#
Use the following IOS commands to open the fast ethernet interface Fa0/0 configuration mode on Router02 and configure IP address as 172.18.0.1/16.
Router02>enable  Router02#configure terminal Enter configuration commands, one per line.  End with CNTL/Z. Router02(config)#interface fa0/0 Router02(config-if)#ip address 172.18.0.1 255.255.0.0 Router02(config-if)#no shutdown
Use the following IOS commands to open the serial interface S0/0 configuration mode on Router02 and configure IP address as 172.17.0.2/16.
Router02>enable Router02#configure terminal Enter configuration commands, one per line.  End with CNTL/Z. Router02(config)#interface s0/0 Router02(config-if)#ip address 172.17.0.2 255.255.0.0 Router02(config-if)#no shutdown
Use the following IOS commands to open the serial interface S0/1 configuration mode on Router02 and configure IP address as 172.19.0.1/16. You have to set a clock rate also using the "clock rate" command on S0/1 interface, since this is the DCE side.
Router02>enable Router02#configure terminal Enter configuration commands, one per line.  End with CNTL/Z. Router02(config)#interface s0/1 Router02(config-if)#clock rate 64000 Router02(config-if)#ip address 172.19.0.1 255.255.0.0 Router02(config-if)#no shutdown
Do remember to run the "copy running-config startup-config" command from enable mode, if you want to save the changes you have made in the router.
Hostname and IP address configuration in Router03
Connect to Router03 console and use the following IOS commands to configure host name as Router03.
Router>enable Router#configure terminal Enter configuration commands, one per line.  End with CNTL/Z. Router(config)#hostname Router03 Router03(config)#
Use the following IOS commands to open the fast ethernet interface Fa0/0 configuration mode on Router03 and configure IP address as 172.20.0.1/16.
Router03>enable Router03#configure terminal Enter configuration commands, one per line.  End with CNTL/Z. Router03(config)#interface fa0/0
Router03(config-if)#ip address 172.20.0.1 255.255.0.0 Router03(config-if)#no shutdown
Use the following IOS commands to open the serial interface S0/1 configuration mode on Router03 and configure IP address as 172.19.0.2/16.
Router03>enable Router03#configure terminal Enter configuration commands, one per line.  End with CNTL/Z. Router03(config)#interface s0/1 Router03(config-if)#ip address 172.19.0.2 255.255.0.0 Router03(config-if)#no shutdown
Do remember to run the "copy running-config startup-config" command from enable mode, if you want to save the changes you have made in the router.
Enhanced Interior Gateway Routing Protocol (EIGRP) configuration in Router01
Connect to Router01 console and use the following IOS commands to configure Enhanced Interior Gateway Routing Protocol (EIGRP) in Router01. Please refer the beginning of this lesson to view the Enhanced Interior Gateway Routing Protocol (EIGRP) configuration IOS command.
In the IOS "network" command, shown below, we specify only the directly connected networks of this router.
Router01>enable  Router01#configure terminal Enter configuration commands, one per line.  End with CNTL/Z. Router01(config)#router eigrp 1 Router01(config-router)#network 172.16.0.0 Router01(config-router)#network 172.17.0.0 Router01(config-router)#exit
Router01(config)#exit Router01#
Do remember to run the "copy running-config startup-config" command from enable mode, if you want to save the changes you have made in the router.
Enhanced Interior Gateway Routing Protocol (EIGRP) configuration in Router02
Connect to Router02 console and use the following IOS commands to configure Enhanced Interior Gateway Routing Protocol (EIGRP) in Router02. Please refer the beginning of this lesson to view the Enhanced Interior Gateway Routing Protocol (EIGRP) configuration IOS command.
In the IOS "network" command, shown below, we specify only the directly connected networks of this router.
Router02>enable Router02#configure terminal Enter configuration commands, one per line.  End with CNTL/Z. Router02(config)#router eigrp 1 Router02(config-router)#network 172.17.0.0 Router02(config-router)#network 172.18.0.0 Router02(config-router)#network 172.19.0.0 Router02(config-router)#exit Router02(config)#exit Router02#
Do remember to run the "copy running-config startup-config" command from enable mode, if you want to save the changes you have made in the router.
Enhanced Interior Gateway Routing Protocol (EIGRP) configuration in Router03
Connect to Router03 console and use the following IOS commands to configure Enhanced Interior Gateway Routing Protocol (EIGRP) in Router03. Please refer the beginning of this
lesson to view the Enhanced Interior Gateway Routing Protocol (EIGRP) configuration IOS command.
In the IOS "network" command, shown below, we specify only the directly connected networks of this router.
Router03>enable Router03#configure terminal Enter configuration commands, one per line.  End with CNTL/Z. Router03(config)#router eigrp 1 Router03(config-router)#network 172.19.0.0 Router03(config-router)#network 172.20.0.0 Router03(config-router)#exit Router03(config)#exit Router03#
Do remember to run the "copy running-config startup-config" command from enable mode, if you want to save the changes you have made in the router.
How to view the routing table in Router01
After the initial configuration and Enhanced Interior Gateway Routing Protocol (EIGRP) configuration in all routers, we can use the "show ip route" to view the routing table in Router01