• atulsharma.cse@gmail.com
• January 12, 2023
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Known as the Enhanced Interior Gateway Routing Protocol (EIGRP), it is a hybrid routing protocol. In this blog, we will learn about the EIGRP protocol and how it works.

In the Previous blog, we learned about the Routing Information  protocol (RIP), a distance-vector interior gateway protocol. I definitely recommend you go through that blog before Jumping to this blog. 

Here, we are about to learn about the EIGRP protocol and how it works. we will know how EIGRP chooses the best route to send the packets. After that, we will also learn about the different packet types that the EIGRP uses for the management of the sessions. At last, we will learn a few benefits of the EIGRP.

Without any further ado, let us begin learning about EIGRP in networking in detail!

What is Meant By EIGRP?

You will find out that many things about EIGRP are similar to RIP. Let us begin learning about some key features of EIGRP:

• The term EIGRP is an acronym that stands for Enhanced Interior Gateway Routing Protocol.
• It is an improved version of the older Interior Gateway Routing Protocol (IGRP).
• EIGRP was previously a Cisco-owned protocol. However, Cisco has now made it an open-standard protocol. Therefore, any vendor can use it now on their equipment!
• EIGRP is considered an ‘advanced’ or ‘hybrid’ distance vector routing protocol.
• It improves many basic operations of RIP which is its fellow distance vector routing protocol.
• It is way faster than RIP in reacting to the changes needed in the network.
• It does not have a  hop count limit of 15 like RIP. Therefore, it supports very large networks.
• It sends messages using the multicast address 224.0.0.10.
• EIGRP can be deployed on both IPv4 and IPv6 Internet Protocol networks. It can also be deployed on networks such as Novell Internetwork Packet Exchange.
• EIGRP is the only Interior Gateway Protocol (IGP) that can perform unequal-cost-load-balancing. It performs equal-cost-load-balancing (ECMP) load balancing over 4 paths like RIP by default.
• EIGRP can even load balance on the basis of the proportion of their bandwidth. Therefore, more traffic will be sent over to the paths with lower metric as they are faster.
• Less traffic will be sent to the paths with higher metric as they are slower.

EIGRP is a wonderful protocol. However, it is mostly used in Cisco devices, therefore, it is not as widely used as OSPF.

How Does The EIGRP Choose The Best Path?

Unlike other routing protocols, EIGRP uses four major metrics to choose the best path to transmit the packets. These metrics are:

• Bandwidth
• Delay
• Load, and 
• Reliability

The default metric used by EIGRP is bandwidth and delay. 

EIGRP uses the following table for route discovery:

• Neighbor table
• Topology table
• Route table
• Successors
• Feasible successors

Now, let’s see how this table works step-by-step.

1. Neighbor Table:

• The neighbor table is used to store the condition of the directly connected neighbors.
• This means that whether the directly connected router is active or non-active, the neighbor table is going to store the information anyway.

For example, if two routers are connected to each other directly, both routers are going to send the ‘Hello’ packets to each other after every 5 seconds. These two routers consider each other alive as long as this exchange of packets continues.

If the router is not able to receive the hello packet in 5 seconds, it realizes that the link is down and forwards this information to another connected router. In such a way, all the routers are updated about the link failure of a particular router in the network.

2. Topology Table:

• This table is used to store the advertisements of the destinations by the neighbors.
• The advertisement includes:
  • The destination address
  • The neighbor that is linked to the destination
  • The metrics used by the neighbor to the destination

For example, if three routers are connected to each other in a linear form and the first router wants to reach the third router, it is not going to receive any advertisement from the third router. In fact, it will receive an advertisement from the second router. 

This is because the second router knows the best route to reach the third router and the first router knows the best route to reach the second router. This is how all the routers are configured with EIGRP to calculate and update the router to their routing table.

3. Routing Table:

• This table is used to store all the possible routes to reach the destination.
• This is the main function of the routing table.

4. Successors:

• The successor is the path with the best metric.
• This means that the successor is the best available route in the routing table to reach the destination and the router chooses this route first.

5. Feasible Successors:

• It is considered a backup route.
• If the path distance is less than the original path to reach the destination, it is considered feasible.
• When the successor path is invalid or inactive, it becomes the first choice.
• It is the second fastest available route to reach the destination.

The algorithm which determines the most efficient and reasonable routes to reachable destinations is the Diffusing Update Algorithm (DUAL). This is decided on the basis of the distance and whether the destination path is loop-free.

What Packages are Used By EIGRP?

EIGRP uses 5 packages to pass messages and allow the management of the sessions. These are:

• HELLO packets
• QUERY packets
• REPLY packets
• REQUEST packets
• UPDATE packets

What are the Benefits of EIGRP?

The EIGRP protocol has the following benefits:

• It has a faster convergence.
• There is no need to update the routing tables by hand.
• It has a low network resource usage as it only sends HELLO packets when the network is stable.
• It either uses unequal-cost-load-balancing or equal-cost multipath balancing to use links more efficiently.
• It is loop-free therefore, it improves video and voice quality.

Conclusion

EIGRP is the new-age Interior Gateway Protocol (IGP) that has a faster convergence. Its highlighting feature is the unequal-cost-load-balancing that in return, reduces CPU usage.

However, EIGRP is more of a Cisco proprietary even though some of its features are open-standard. OSPF is considered over EIGRP!

Stay tuned for learning more Protocols such as BGP in the CCNA series

Happy learning!

https://youtu.be/BNxTdIcypTQ