EIGRP is a Advanced distance Vector routing protocol, as we know RIP uses concept of Hop count to find the best path to a specific route and OSPF uses Link properties to find the best path for a specific route, So Here also EIGRP uses both kind of mechanism to install the route in Routing table. For more info: https://corenetworkingclasses.com/EIGRPZerotoHero.php

1. EIGRP
ZERO TO HERO
Enhanced Interior Gateway Routing Protocol

2. Properties of EIGRP
Whenever a EIGRP router gets a route from internal AS Routers it use AD
90 to install routes in Routing table
• Whenever a EIGRP router gets a route from external AS or other domain
routes using redistribution then routers in EIGRP install routes in routing
table with AD 170
• EIGRP uses concept of Autonomous number AS to run EIGRP, AS
number size is 16 bit and we can have a range from 1 to 65535 to
configure on a router.
• EIGRP use timers
Hello: 5 Sec, in every 5 sec router sends a hello to all active eigrp
interfaces
Hold: 15 Sec, routers consider a neighbor relationship down if
did not get hello packet till next 15sec of first Hello received.

3. Messages
Messages
EIGRP uses 5
different Messages
HELLO
UPDATE
ACK
QUERY
REPLY

4. Let’s enable EIGRP between two routers and will understand the
concept of Neighborship between two EIGRP routers
Neighborship Process
*R1(config)#router eigrp 10
*R1(config-router)#network 12.0.0.0
*R1(config-router)#exit
*R2(config)#router eigrp 10
*R2(config-router)#network 12.0.0.0
*R2(config-router)#exit

5. EIGRP uses a classic metric calculation formula
*It use total Delay throughout the destination
*least bandwidth in between source to destination to find the metric
*Routers always use the receiving end link properties to do the metric and calculation
EIGRP Metric
Calculation Process

6. Static Neighborship
By default routers use multicast address 224.0.0.10 to discover neighbor and form
the dynamic neighborship, so whenever they get any random packet and find’s the
same AS number and K values then they form the neighborship.
But we can secure our network by using static neighborship in EIGRP, when we
use static neighborship concept so neighbor commands should be configured on
both routers to form EIGRP neighborship.
In static neighborship Network command is used to run EIGRP over the interface
and neighbor command Is used to form one to one neighborship.

7. Auto-Summary
Feature
Auto-summary feature is default enable in
OS Version 12 & is disable in new Version
15.2 & other latest release
So we will discuss on Version 15 and on
latest release.

8. Metric Stuff
(CD, FD & AD)
Now let’s discuss more details on metric and other
ingredients used in metric
To reach Router R3 loopback address from R1 we
have 3 paths available, on is direct to R3, other is via
R2 and last one is via R4
First of all router calculate metric of all available
path and that is called as CD (Computed distance)
*R1 to R3 is 156160
*R1 to R3 via R2 is 156416
*R1 to R3 via R4 is 158720