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EIGRP – Part 1

Author: gokhankosem, on 02 Oct 16 - 0 Comments
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(Enhanced Interior Gateway
Routing Protocol)

EIGRP (Enhanced Interior Gateway Routing Protocol) is a It is a hybrid routing protocol that show both distance vector and link-state routing protocol properties. EIGRP (Enhanced Interior Gateway Routing Protocol) was developed by Cisco as a Cisco proprietary protocol and it is only available on Cisco devices.

EIGRP (Enhanced Interior Gateway Routing Protocol) is a classless routing protocol. It sends the subnet masks in the update packets. EIGRP also supports VLSM (Variable Length Subnet Mask), so EIGRP uses the IP addresses range very efficent.

The most important characteristic of EIGRP is that, it is a fast converged routing protocol. EIGRP keeps the “Backup Routes” beside the “Best Route” to a destination. So, during a failure, EIGRP does not need to calculate a new route. Instead, EIGRP uses the Backup Route. This provides to EIGRP, Faster Convergence mechanism.

EIGRP (Enhanced Interior Gateway Routing Protocol) uses a very complex metric by using bandwith, delay, load, reliability and MTU values. This formule is showed below:

EIGRP Configuration with Packet Tracer

EIGRP Metric Formula

As you can see, there are some K values in EGRP metric formule. By default these values are:

K1 (Bandwidth) = 1
K2 (Load) = 0
K3 (Delay) = 1
K4(Reliability) = 0
K5(MTU) = 0

These K values need to be match in both ends to form an EIGRP Neighbourship. You can change these values but it is not recommended.

By default Bandwith and Delay values become important only and used as a metric ( The other parts will be 0, becaue of multiplying with 0). So, we can say that, EIGRP (Enhanced Interior Gateway Routing Protocol) uses the path with minimum bandwidth and the total delay to a destination.

Bandwidth = 107/Bandwidth in Kbps
Delay = Delay / 10

The defalut Formula becomes like below (with default K values ) :

EIGRP Configuration with Packet Tracer

EIGRP Default Metric

EIGRP (Enhanced Interior Gateway Routing Protocol) works with IPv4, IPv6, IPX ans AppleTalk Routed Protocols. So with this characteristic, we can say that EIGRP is a protocol independant routing protocol.

EIGRP uses DUAL (Diffusing Update Algorithm) for routing calculations.

The Administrative Distance for Internal EIGRP is 90, for External EIGRP is 170 and for Summary Advertisemet is 5.

EIGRP uses RTP (Reliable Transport Protocol) to control sending and acknowledging EIGRP messages.

The multicast address used for EIGRP (Enhanced Interior Gateway Routing Protocol) is The protocol number of EIGRP is 88.

The default Hello Timer is 5 seconds and the Hold-Down Timer is 15 seconds in EIGRP. This is a little more for NBMA (Non Broadcast Multi Access) networks . For NBMA networks, Hello Timer is 60 seconds and Hold-Down Timer is 180 seconds. In OSPF the Hello and Dead intervals must be match to form neighbourship. But in EIGRP Hello and Hold-Down timers can be different and it is not a necessity for neighbourship.

In EIGRP, MD5 authentication mechanism is used. EIGRP does not use simple authentication.

EIGRP Article Series…
EIGRP – Part 1
EIGRP – Part 2 (EIGRP Tables)
EIGRP – Part 3 (EIGRP Packet Types and Neighborship)
EIGRP – Part 4 (Other EIGRP Properties)

EIGRP Configuration with Packet Tracer

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About the Author
Gokhan Kosem is a telecommunation and network engineer. His ambition to IP networks and end-to-end system installation made him to prepare this web-site. By sharing his experiences about various networking protocols beside different system installation experiences and Cisco, Juniper, Alcatel-Lucent devices configurations, he is aimed to be helpful for his collegues in all over the world. He is currently lives in Istanbul, Turkey.

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