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🌐 EIGRP (Enhance Interior Gateway Routing Protocol)

 

📖 Introduction

In dynamic routing, different routing protocols are used to allow routers to communicate and share routing information, and among these protocols, EIGRP is an advanced routing protocol that provides faster convergence and better performance compared to basic protocols like RIP, which makes it suitable for medium to large networks where efficiency, speed, and reliability are important.

 

📖 Definition

EIGRP (Enhanced Interior Gateway Routing Protocol) is a dynamic routing protocol that allows routers to automatically share routing information and select the best path based on multiple factors such as bandwidth, delay, reliability, and load instead of just hop count.

 

🧠 Basic Information

  • EIGRP → Enhanced Interior Gateway Routing Protocol 
  • EIGRP was developed by Cisco 
  • It was earlier a Cisco proprietary protocol, but later it was partially released as an open standard 
  • It is suitable for medium-sized networks 

 

🔍 Concept Explanation

EIGRP is more advanced than RIP because it does not depend only on hop count, and instead, it uses multiple metrics to calculate the best path, which makes routing more efficient and intelligent.

👉 Key concept:

  • Routers build a neighbor relationship called adjacency 
  • Once adjacency is formed, routers begin to share routing information 
  • Routing becomes faster and more reliable 

 

📌 Adjacency (Neighbor Relationship)

In EIGRP, routers must first form a neighbor relationship (adjacency), and only after this relationship is successfully established, routers begin to exchange routing information with each other, which ensures controlled and efficient communication.

 

📡 Autonomous System (AS) Concept

EIGRP uses the concept of Autonomous System (AS):

  • An AS is a group of routers sharing the same routing information 
  • Each AS is identified using an ASN (Autonomous System Number) 

 

🔢 ASN Details

  • ASN is a 16-bit number 
  • Range: 1 – 65535 (0 is reserved) 

 

🔹 Types of ASN

  • Public ASN → 1 – 64511 
  • Private ASN → 64512 – 65535 

👉 Note:

  • ASN 0 is reserved for research and experimental purposes 

 

📦 Types of EIGRP Packets

EIGRP uses 5 types of packets:

  • Hello Packet → Used to discover neighbors 
  • Update Packet → Used to share routing information 
  • Query Packet → Used when route is lost 
  • Reply Packet → Response to query 
  • Acknowledgement (ACK) Packet → Confirms receipt 

 

📊 Metrics Used in EIGRP

EIGRP selects the best path using:

  • Bandwidth  
  • Delay 
  • Reliability  
  • Load 

👉 This makes it more efficient than RIP.

 

📏 Hop Count Limit

  • Default hop count = 100 
  • Maximum hop count = 255 

 

📚 Tables Maintained by EIGRP

EIGRP maintains 3 important tables:

1️⃣ Neighbor Table

👉 Stores information about directly connected neighbor routers

2️⃣ Topology Table

👉 Stores information about all possible routes (primary and backup routes)

3️⃣ Routing Table

👉 Stores only the best routes (shortest path)

 

🔢 Wildcard Mask in EIGRP

EIGRP uses wildcard mask, which is the opposite of subnet mask.

📌 Formula

👉 Wildcard Mask =
255.255.255.255 – Subnet Mask

 

📊 Default Wildcard Masks

  • Class A → 0.255.255.255 
  • Class B → 0.0.255.255 
  • Class C → 0.0.0.255 

 

⚠️ Important Note

  • In Cisco routers, subnet mask can also be used 
  • Router automatically converts it to wildcard mask 
  • In other vendors (like Juniper), wildcard mask must be used 

 

✨ Key Features of EIGRP

  • Uses advanced metric (not only hop count) 
  • Faster convergence 
  • Supports automatic route updates 
  • Maintains backup routes 
  • Efficient use of bandwidth 
  • Uses adjacency for communication 

 

✅ Advantages of EIGRP

  • It provides faster convergence, which means it quickly updates routes when there is any change in the network 
  • It selects the best path using multiple factors, which improves overall network performance 
  • It supports backup routes (feasible successor), which increases reliability 
  • It uses bandwidth efficiently and reduces unnecessary traffic 
  • It is suitable for medium to large networks 

 

❌ Disadvantages of EIGRP

  • It is more complex compared to basic protocols like RIP 
  • It requires proper understanding for configuration 
  • It is mainly used in Cisco environments 
  • It consumes more resources than simple routing protocols 

 

⚠️ Important Understanding

EIGRP is a powerful routing protocol that combines fast performance, intelligent routing, and reliability, and although it is more complex than basic protocols, it is widely used in real-world networks because of its efficiency and ability to handle network changes quickly.

 

🎯 Key Idea

👉 EIGRP = Fast + Smart + Reliable Routing
👉 Best path = Based on multiple factors (not just hops)


 

🖧 EIGRP Configuration (Syntax & Steps)

 

📖 Introduction

After understanding EIGRP as an advanced dynamic routing protocol, the next step is to configure it on routers so that they can automatically exchange routing information and select the best path, and this configuration is done using Cisco IOS commands where we enable EIGRP with an Autonomous System Number and specify the networks that will participate in routing.

 

📌 EIGRP Configuration Syntax

The basic syntax to configure EIGRP is:

router eigrp <AS number>
network <network-address>
no auto-summary

 

🔍 Explanation of Syntax

  • router eigrp <AS number> → Enables EIGRP with a specific Autonomous System Number 
  • network <network-address> → Specifies the networks that will participate in EIGRP 
  • no auto-summary → Disables automatic summarization (recommended) 

 

⚠️ Important Points

  • All routers must have the same AS number 
  • Only directly connected networks should be added 
  • Interfaces must be active (no shutdown) 

 

📊 Example

router eigrp 1
network 192.168.1.0
network 192.168.2.0
no auto-summary

👉 Meaning:

  • EIGRP is enabled with AS number 1 
  • Router will advertise both networks 

 

🔄 (Optional) Using Wildcard Mask

👉 EIGRP can also use wildcard mask (optional):

router eigrp 1
network 192.168.1.0 0.0.0.255

✔ Used for more precise control
❗ Not required for basic configuration

 

📌 Steps to Configure EIGRP

🧠 Concept

Before configuring EIGRP:

  • Assign IP addresses to all interfaces 
  • Ensure interfaces are up 
  • Identify all network addresses 

 

🔧 Step-by-Step Configuration

 

1️⃣ Enter Privileged Mode

Router> enable

 

2️⃣ Enter Global Configuration Mode

Router# configure terminal

 

3️⃣ Enable EIGRP with AS Number

Router(config)# router eigrp 1

 

4️⃣ Add Networks

Router(config-router)# network 192.168.1.0
Router(config-router)# network 192.168.2.0

👉 Add all directly connected networks

 

5️⃣ Disable Auto Summary

Router(config-router)# no auto-summary

 

6️⃣ Exit Configuration Mode

Router(config-router)# exit
Router(config)# end

 

7️⃣ Verify Configuration

Router# show ip route

👉 Check routing table

Router# show ip protocols

👉 Check EIGRP details

 

📊 Complete Example

Router> enable
Router# configure terminal
Router(config)# router eigrp 1
Router(config-router)# network 192.168.1.0
Router(config-router)# network 192.168.2.0
Router(config-router)# no auto-summary
Router(config-router)# end
Router# show ip route

 

✨ Key Features of EIGRP Configuration

  • Simple configuration with AS number 
  • Automatic route sharing 
  • Faster convergence 
  • Supports large networks 
  • No need for manual route entry 

 

⚠️ Important Notes

  • AS number must match on all routers 
  • Use correct network addresses 
  • Use no auto-summary 
  • Configure on all routers 

 

🎯 Key Idea

👉 EIGRP configuration =
Enable EIGRP + Add networks + Same AS → Routers automatically share routes


 

🖧 EIGRP Configuration in a Four-LAN Network

 

🌐 Introduction

In this practical, we have implemented dynamic routing using EIGRP (Enhanced Interior Gateway Routing Protocol) in a four-LAN topology, where routers automatically exchange routing information and select the best path based on multiple metrics such as bandwidth and delay, which makes the network faster and more efficient compared to RIP and static routing.

 

🖼️ Network Topology

👉 The network consists of:

  • LAN 1 → 192.168.100.0/24 
  • LAN 2 → 192.168.101.0/24 
  • LAN 3 → 192.168.102.0/24 
  • LAN 4 → 192.168.103.0/24 

👉 Routers used:

  • GATEWAY1  
  • ROUTER1  
  • GATEWAY2  

 

📡 Working Concept

In EIGRP, routers first form a neighbor relationship (adjacency), and once the adjacency is established, they begin to exchange routing information, and based on this information, they build their routing tables and select the best path, and if any network change occurs, EIGRP quickly updates the routes to maintain smooth communication.

 

⚙️ Router Configurations

 

🔹 GATEWAY1 CONFIGURATION

Router>ENABLE
Router#CONFIG T
Router(config)#HOSTNAME GATEWAY1

GATEWAY1(config)#INTERFACE FA0/0
GATEWAY1(config-if)#IP ADDRESS 192.168.100.4 255.255.255.0
GATEWAY1(config-if)#NO SHUTDOWN
GATEWAY1(config-if)#EXIT

GATEWAY1(config)#INTERFACE FA0/1
GATEWAY1(config-if)#IP ADDRESS 192.168.101.1 255.255.255.0
GATEWAY1(config-if)#NO SHUTDOWN
GATEWAY1(config-if)#EXIT

GATEWAY1(config)#ROUTER EIGRP 65111
GATEWAY1(config-router)#NETWORK 192.168.100.0 0.0.0.255
GATEWAY1(config-router)#NETWORK 192.168.101.0 0.0.0.255
GATEWAY1(config-router)#NO AUTO-SUMMARY
GATEWAY1(config-router)#EXIT

GATEWAY1(config)#EXIT

 

🔹 ROUTER1 CONFIGURATION

Router>ENABLE
Router#CONFIG T
Router(config)#HOSTNAME ROUTER1

ROUTER1(config)#INTERFACE FA0/0
ROUTER1(config-if)#IP ADDRESS 192.168.101.2 255.255.255.0
ROUTER1(config-if)#NO SHUTDOWN
ROUTER1(config-if)#EXIT

ROUTER1(config)#INTERFACE FA0/1
ROUTER1(config-if)#IP ADDRESS 192.168.102.1 255.255.255.0
ROUTER1(config-if)#NO SHUTDOWN
ROUTER1(config-if)#EXIT

ROUTER1(config)#ROUTER EIGRP 65111
ROUTER1(config-router)#NETWORK 192.168.101.0 0.0.0.255
ROUTER1(config-router)#NETWORK 192.168.102.0 0.0.0.255
ROUTER1(config-router)#NO AUTO-SUMMARY
ROUTER1(config-router)#EXIT

ROUTER1(config)#EXIT

 

🔹 GATEWAY2 CONFIGURATION

Router>ENABLE
Router#CONFIG T
Router(config)#HOSTNAME GATEWAY2

GATEWAY2(config)#INTERFACE FA0/0
GATEWAY2(config-if)#IP ADDRESS 192.168.103.1 255.255.255.0
GATEWAY2(config-if)#NO SHUTDOWN
GATEWAY2(config-if)#EXIT

GATEWAY2(config)#INTERFACE FA0/1
GATEWAY2(config-if)#IP ADDRESS 192.168.102.2 255.255.255.0
GATEWAY2(config-if)#NO SHUTDOWN
GATEWAY2(config-if)#EXIT

GATEWAY2(config)#ROUTER EIGRP 65111
GATEWAY2(config-router)#NETWORK 192.168.102.0 0.0.0.255
GATEWAY2(config-router)#NETWORK 192.168.103.0 0.0.0.255
GATEWAY2(config-router)#NO AUTO-SUMMARY
GATEWAY2(config-router)#EXIT

GATEWAY2(config)#EXIT

 

🖼️ Connectivity Test

👉 From the image:

  • PC/Laptop successfully pings 192.168.103.2 
  • Replies received with 0% packet loss 

👉 This confirms:
✔ All networks are reachable
✔ EIGRP is working correctly

 

📊 Routing Table Verification

👉 Use command:

show ip route

 

🎯 Explanation of Output

👉 In routing table:

  • D → Routes learned via EIGRP 
  • C → Directly connected networks 

👉 This confirms:
✔ Routers are exchanging routes
✔ EIGRP is functioning properly

 

🎯 Key Understanding

  • No need to configure routes manually 
  • Routers automatically learn all networks 
  • EIGRP selects best path using advanced metrics 
  • All routers must use same AS number 

 

⚠️ Important Points

  • AS number must be same on all routers 
  • Add only directly connected networks 
  • Use no auto-summary 
  • Interfaces must be active 

 

🎯 Key Idea

👉 EIGRP configuration =
Enable EIGRP + Same AS + Add networks → Automatic smart routing

 

🎯 Summary

In this four-LAN topology, EIGRP is configured to enable dynamic routing where routers automatically exchange routing information and select the best path, and as a result, all networks can communicate efficiently with faster convergence and better performance compared to RIP and static routing.

 

👉EIGRP Interview Questions:

 

❓ 1️⃣ What is EIGRP?

👉 Answer:
EIGRP (Enhanced Interior Gateway Routing Protocol) is a dynamic routing protocol that automatically shares routing information and selects the best path using multiple metrics like bandwidth, delay, reliability, and load.

 

❓ 2️⃣ What metrics does EIGRP use to select the best path?

👉 Answer:
EIGRP uses the following metrics:

  • Bandwidth
  • Delay
  • Reliability
  • Load

👉 By default, bandwidth and delay are mainly used.

 

❓ 3️⃣ What is adjacency in EIGRP?

👉 Answer:
Adjacency is the neighbor relationship formed between routers.
Routers must form adjacency before exchanging routing information.

 

❓ 4️⃣ What are the tables used in EIGRP?

👉 Answer:
EIGRP maintains three tables:

  • Neighbor Table
  • Topology Table
  • Routing Table

 

❓ 5️⃣ What is the purpose of Autonomous System (AS) in EIGRP?

👉 Answer:
The AS number is used to identify a group of routers running EIGRP.
All routers must have the same AS number to exchange routing information.