EIGRP Explained: Enhanced Interior Gateway Routing Protocol – Configuration, Working, Metrics and Interview Questions
Introduction to EIGRP
EIGRP (Enhanced Interior Gateway Routing Protocol) is an advanced distance-vector routing protocol developed by Cisco. It is designed to help routers dynamically exchange routing information and identify the best paths to destination networks.
EIGRP is widely used in enterprise networking because of its fast convergence, efficient routing updates, and flexible metric calculation.
If you are preparing for CCNA, CCNP, or a Network Engineer interview, understanding EIGRP is an important part of learning dynamic routing protocols.
In this blog, we will explore EIGRP fundamentals, how it works, neighbor adjacency requirements, metric calculation, basic configuration, verification commands, and troubleshooting.
1. What is EIGRP?
EIGRP stands for Enhanced Interior Gateway Routing Protocol.
It is an Interior Gateway Protocol (IGP), which means it is used to exchange routing information within an autonomous system.
Important characteristics:
Advanced distance-vector routing protocol.
Uses DUAL (Diffusing Update Algorithm).
Supports IPv4 and IPv6.
Supports unequal-cost load balancing.
Provides fast convergence.
Uses incremental routing updates.
Supports route summarization.
Uses IP Protocol Number 88.
2. How Does EIGRP Work?
EIGRP routers exchange Hello packets to discover neighboring routers.
Once neighbors are discovered, they establish adjacency and exchange routing information.
EIGRP uses three important tables:
A. Neighbor Table
Contains information about directly connected EIGRP neighbors.
B. Topology Table
Contains routes learned from EIGRP neighbors, including successor and feasible successor information.
C. Routing Table
Contains the best routes selected by EIGRP and other routing protocols.
3. EIGRP Neighbor Adjacency Requirements
For two routers to establish an EIGRP neighbor relationship, the following conditions must be satisfied:
Both routers must use the same EIGRP Autonomous System (AS) number.
K-values must match.
IP connectivity must exist on the common link.
EIGRP must be enabled on the connecting interfaces.
Authentication parameters must match, if authentication is configured.
EIGRP must not be configured as passive on the connecting interface.
The routers must exchange compatible EIGRP Hello packets.
Important: The network statements do not need to be identical on both routers. They must enable EIGRP on the common interface.
4. EIGRP Packet Types
EIGRP uses five packet types:
| Packet Type | Purpose |
|---|---|
| Hello | Discovers and maintains neighbors |
| Update | Exchanges routing information |
| Query | Requests alternative routing information |
| Reply | Responds to Query packets |
| Acknowledgment | Acknowledges reliable EIGRP packets |
EIGRP uses RTP (Reliable Transport Protocol) to provide reliable delivery of certain EIGRP packets.
5. EIGRP Metric Calculation
EIGRP uses a composite metric to select the best path.
By default, the metric calculation considers:
Bandwidth
Delay
EIGRP also supports additional metric components:
Reliability
Load
The default K-values are:
| K-value | Value | Metric Component |
|---|---|---|
| K1 | 1 | Bandwidth |
| K2 | 0 | Load |
| K3 | 1 | Delay |
| K4 | 0 | Reliability |
| K5 | 0 | Additional reliability-related factor |
The default metric formula is based on minimum bandwidth and cumulative delay.
6. EIGRP Administrative Distance
Administrative Distance (AD) determines the trustworthiness of a routing source.
| Route Type | Administrative Distance |
|---|---|
| Internal EIGRP | 90 |
| External EIGRP | 170 |
| EIGRP Summary Route | 5 |
Lower Administrative Distance is preferred when multiple routing sources advertise the same destination prefix.
7. Basic EIGRP Configuration Between Two Routers
Let's understand EIGRP with a practical Cisco Packet Tracer example.
Network Topology
Two routers are connected through an inter-router network.
IP Addressing Scheme
| Device | Interface | IP Address |
|---|---|---|
| R1 | G0/0 | 192.168.1.1/24 |
| R1 | G0/1 | 10.0.0.1/30 |
| R2 | G0/0 | 192.168.2.1/24 |
| R2 | G0/1 | 10.0.0.2/30 |
Router R1 Configuration
enable
configure terminal
hostname R1
interface gigabitEthernet 0/0
ip address 192.168.1.1 255.255.255.0
no shutdown
exit
interface gigabitEthernet 0/1
ip address 10.0.0.1 255.255.255.252
no shutdown
exit
router eigrp 100
network 192.168.1.0
network 10.0.0.0
no auto-summary
exit
end
write memoryRouter R2 Configuration
enable
configure terminal
hostname R2
interface gigabitEthernet 0/0
ip address 192.168.2.1 255.255.255.0
no shutdown
exit
interface gigabitEthernet 0/1
ip address 10.0.0.2 255.255.255.252
no shutdown
exit
router eigrp 100
network 192.168.2.0
network 10.0.0.0
no auto-summary
exit
end
write memoryUnderstanding the EIGRP Commands
router eigrp 100
Enables EIGRP using Autonomous System number 100.
network 192.168.2.0
Enables EIGRP on interfaces matching the specified network and advertises the connected network.
network 10.0.0.0
Enables EIGRP on the interface connecting the two routers.
no auto-summary
Disables automatic classful route summarization, preserving more specific network prefixes.
exit
Exits EIGRP configuration mode.
8. EIGRP Verification Commands
After configuring EIGRP, use these commands to verify the configuration.
| Command | Purpose |
|---|---|
| show ip eigrp neighbors | Displays EIGRP neighbors |
| show ip eigrp topology | Displays EIGRP topology |
| show ip route eigrp | Displays EIGRP-learned routes |
| show ip protocols | Displays routing protocol information |
| show ip eigrp interfaces | Displays EIGRP-enabled interfaces |
| show ip interface brief | Displays interface status |
In the routing table, EIGRP-learned internal routes are identified by the letter D.
9. What is DUAL in EIGRP?
DUAL stands for Diffusing Update Algorithm.
It is the algorithm used by EIGRP to calculate loop-free paths and provide fast convergence.
Two important EIGRP route concepts are:
Successor: The best path to a destination, installed in the routing table.
Feasible Successor: A loop-free backup path that satisfies the Feasibility Condition.
The Feasibility Condition is satisfied when the reported distance from a neighbor is less than the current feasible distance.
This allows EIGRP to use an eligible backup path without performing a new route calculation.
10. EIGRP vs OSPF
| Feature | EIGRP | OSPF |
|---|---|---|
| Protocol Type | Advanced Distance Vector | Link State |
| Algorithm | DUAL | SPF |
| Metric | Composite Metric | Cost |
| AD | 90 Internal | 110 |
| IP Protocol | 88 | 89 |
| Multicast IPv4 | 224.0.0.10 | 224.0.0.5 and 224.0.0.6 |
| Load Balancing | Equal and Unequal Cost | Equal Cost |
| Convergence | Generally Fast | Generally Fast |
11. Common EIGRP Interview Questions
Q1. What is the full form of EIGRP?
Enhanced Interior Gateway Routing Protocol.
Q2. What is the default Administrative Distance of internal EIGRP?
Q3. What is the default Administrative Distance of external EIGRP?
Q4. Which algorithm does EIGRP use?
DUAL (Diffusing Update Algorithm).
Q5. What is the IP protocol number of EIGRP?
Q6. What is the multicast address used by EIGRP for IPv4?
224.0.0.10.
Q7. What are the default EIGRP K-values?
K1=1, K2=0, K3=1, K4=0, K5=0.
Q8. What is a successor route?
The best path to a destination selected by EIGRP.
Q9. What is a feasible successor?
A loop-free backup path that satisfies the Feasibility Condition.
Q10. What is the purpose of no auto-summary?
It disables automatic classful route summarization.
Conclusion
EIGRP is an important dynamic routing protocol for network engineers. Its DUAL algorithm, fast convergence, efficient routing updates, and support for unequal-cost load balancing make it an interesting protocol to understand.
For practical learning, configure EIGRP in Cisco Packet Tracer, verify neighbor adjacency, inspect the topology table, and test end-to-end connectivity.
Keep Learning. Keep Configuring. Keep Growing!
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