EIGRP Basics and Authentication


EIGRP Characteristics

Advanced Distance Vector Protocol
discover their neighbors using EIGRP Hello packets, form adjacency and exchange the routing databases reliably with their neighbors.
Incremental Updates
Hello packets are transmitted between neighbors to maintain the relationship
VLSM Support
Classless protocol which supports variable length subnet masking VLSM .You must use no auto-summary command to enable it.
Rapid Convergence
With some condition a router keep information with Feasible Successor(s) which are the backup path towards the destination .
EIGRP uses multicast and unicast .Multicast address reserved for EIGRP is
100% Loop Free
With Diffusing Update Algorithm (DUAL) EIGRP guarantees there will not going to be any loops formed
Equal and Unequal Cost Load Balancing
With metric we can perfom load balancing.


EIGRP Databases
Neighbor Table : N
eighbors discovered with an adjacency form and exchanged routing information.

Topology Table :  Lists all the paths with metric and next-hop to all destinations learned.
Advertised Distance AD The best metric a router receives from the neighbor (next-hop device) to a given destination network.
Feasible Distance FD  is the sum of the Advertised Distance and the metric to reach the neighbor.
Feasibility Condition – When the Advertised Distance value  is lower than the best Feasible Distance (metric) to reach the successor:
Feasible Successor FS The second best routes
Successor – The best route to a given destination

Routing Table : Best routes that will be use.


EIGRP Composite Metric
EIGRP uses five different K-values to determine the value of the metric.
K1 – Bandwidth
K2 – Load

K3 – Delay 
K4 – Reliability
K5 – MTU

The default metric it use is Bandwith and Delay .


EIGRP and Wildcard Masks

The AS number of the EIGRP routing process indicates the AS number with which the EIGRP routing process will associate .

The network command indicates that EIGRP will use all interfaces with IP addresses that fall in the classfull network or classless with the use of wildclard masks.

Example configuration :

Config#router eigrp “AS#”
Config-router#network x.x.x.x

Config#router eigrp “AS#”
Config-router#network x.x.x.x x.x.x.x  ( Wildcard Masks )

 EIGRP  Authentication

1st create the keychain
(config)#key chain “name”
(config-keychain)#key “#”
(config-keychain-key)#key-string “name”

2nd apply it to the right interface
(config-if)#ip authentication mode eigrp “AS” md5
(config-if)#ip authentication key-chain eigrp “AS” “chain name”


Veryfying configuration :

Show ip eigrp interfaces ( show interface enable for eigrp as )
Debug eigrp packets ( trying to see anything ) u all to remove debug .
Show ip protocols ( to see on each L3 devices if the 5 K metric are the same )


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