EIGRP (Enhanced Interior Gateway Routing Protocol)

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EIGRP Overview

EIGRP Protocol is one of the most used Dynamic Routing Protocol as IGP (Interior Gateway Protocol). Especially it is used for Routing purpose inside the networks that is mostly consist of Cisco devices.

EIGRP (Enhanced Interior Gateway Routing Protocol) is a Hybrid Routing Protocol that show both Distance Vector and Link-State Routing Protocol properties. It was developed by Cisco as an enhancement to IGRP (Interior Gateway Routing Protocol) and as a Cisco proprietary protocol and it is only available on Cisco devices.


You can test your Routing Knowledge on CCNP Route Practice Tests Page!


EIGRP DUAL Algorithm

 

EIGRP uses DUAL (Diffusing Update Algorithm) for routing calculations. Periodic Updates are not used but if a change occurs in the network like up/down links, update messages are sent.

With DUAL Algotirhm, Topology table is built. In Topology Table both the Best Routes and the Backup Routes to a destination are stored. This provides, a Fast Convergence characteristic, a Fast Convergence Mechanims to EIGRP.

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 in the Topology table. So, during a failure, it does not need to calculate a new route. Instead, it uses the Backup Route towards the destination.


EIGRP Messages

EIGRP uses different types of messages to build neighbourship, to keep the connection and for the updates. These main messages are :

 

  • Hello
  • Update
  • Query
  • Reply
  • Ack

Hello Packets are used to establish and maintain EIGRP Neighbourship.

Update Packets are used to send routing updates.

Query Packets are used to ask for any routing update, requests an update.

Reply Packets are used as a response to the Query Packets.

ACK Packets are used as a feedback to the Update, Query or Reply packets as a feedback mechanism.

We will learn these EIGRP Packets detailly in next lessons.

 

 

EIGRP has three tables. These tables are used on Routing Operation. The EIGRP Tables are given below:

  • Neighbour Table
  • Topology Table
  • Routing Table

Neighbor Table is the table that keeps All EIGRP Neighbour Routers.

Topology Table is the table that keeps All Routes to all the destinations

Routing Table is the table that keeps the Best Routes to any destinations.

 


Classless Protocol

 

Enhanced Interior Gateway Routing Protocol is a Classless Routing Protocol. It sends the subnet masks in the Update Packets.

 


VLSM Support

 

EIGRP also supports VLSM (Variable Length Subnet Mask). Different Subnet Masks can be used with this protocol. By using VLSM, it is a protocol that uses the IP addresses range very efficent.

 


EIGRP Metrics

Enhanced Interior Gateway Routing Protocol uses a very complex metric by using bandwith, delay, load, reliability and MTU values. This EIGRP Metric Formula 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, 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 default Formula becomes like below (with default K values ) :


You can test your Routing Knowledge on CCNP Route Practice Tests Page!


EIGRP Configuration with Packet Tracer
EIGRP Default Metric


Protocol Indemendence

Enhanced Interior Gateway Routing Protocol works with IPv4, IPv6, IPX and AppleTalk Routed Protocols. So with this characteristic, we can say that EIGRP is a Protocol Independent Routing Protocol.


EIGRP Administrative Distance

 

The Administrative Distance (Preference) values for EIGRP are given below:

  • Internal : 90
  • External : 170
  • Summary : 5

If there are routes to a destination with EIGRP, RIP and OSPF, EIGRP (Internal) Routes are prefered over RIP and OSPF Routes. Because the administartive distance of RIP is 120 and OSPF Administrative Distance is 110.

You can find Administrative Distance (Preference) Table for Cisco, Juniper, Nokia and Huawei, here.

 


RTP for Reliable Delivery

 

EIGRP uses RTP (Reliable Transport Protocol) to control sending and acknowledging messages. In other words, RTP ensures delivery of packets. RTP is used by Update, Query and Reply packets. Hello and Ack packets do not use RTP.

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Lesson tags: EIGRP, routing
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