EIGRP Routing Protocol: Design Principles for Enterprise Networks

Author Ermias Teffera

At ITVue Networks, designing scalable and resilient networks is critical. EIGRP (Enhanced Interior Gateway Routing Protocol) is a Cisco-proprietary distance-vector routing protocol optimized for large enterprise networks. In this blog, we’ll explore EIGRP design principles, hierarchical topology considerations, route summarization, and real-world Cisco configurations.

What is EIGRP?

EIGRP is an advanced distance-vector routing protocol that combines the simplicity of distance-vector with the fast convergence of link-state features. Key characteristics:

• Uses DUAL (Diffusing Update Algorithm) for loop-free, rapid convergence

• Supports VLSM (Variable Length Subnet Mask)

• Provides unequal-cost load balancing

• Minimizes unnecessary routing updates

EIGRP Hierarchical Design

Like OSPF, EIGRP benefits from a hierarchical design to improve scalability:

1. Access Layer – Connects end devices to distribution routers

2. Distribution Layer – Aggregates access switches, handles policy, summarization, and redundancy

3. Core Layer – Backbone providing high-speed routing between distribution blocks

Recommended Ratios:

• Access-to-Distribution: 20:1

• Distribution-to-Core: 8:1

These ratios help prevent oversubscription and maintain predictable performance.

Key EIGRP Design Principles

1. Neighbor Relationships

• Establish EIGRP neighbor relationships on all router interfaces within the same AS.

• Use passive interfaces on links that should not form neighbors (e.g., client-facing interfaces).

  
  

2. Route Summarization

• Summarize routes at distribution routers to reduce routing table size.

• Example: 192.168.10.0/24 and 192.168.11.0/24 → 192.168.8.0/21

  
  

3. Bandwidth & Delay Metric Tuning

.

• EIGRP selects the best path based on composite metric: bandwidth, delay, reliability, and load

• Tweak interface bandwidth and delay to influence path selection

4. Unequal-Cost Load Balancing

• By default, EIGRP performs equal-cost load balancing.

• variance command allows traffic over multiple paths with different metrics:

This allows a backup link with a slightly higher metric to carry traffic.

EIGRP Network Diagram

+-------------------+

| Core Router |

+---------+---------+

|

8:1 Ratio

|

+---------+---------+

| Distribution |

+---------+---------+

| |

20:1 Ratio 20:1 Ratio

+-----+ +-----+

|Access| |Access|

+-----+ +-----+

• EIGRP adjacency is formed on all distribution and core routers

• Access routers advertise their networks into distribution

Cisco Configuration Example

• no auto-summary ensures proper handling of discontiguous networks

• passive-interface prevents EIGRP hellos on client-facing ports

Best Practices for EIGRP

• Use hierarchical design to control update propagation

• Summarize routes at distribution and edge to reduce table size

• Monitor metrics using show ip eigrp topology and show ip route eigrp

• Use EtherChannel or LACP for aggregated links to improve bandwidth

• Combine variance for controlled unequal-cost load balancing

Real-World Example

In an enterprise campus:

• Each floor has access switches connecting client devices

• Access routers advertise VLAN networks into distribution EIGRP routers

• Distribution routers summarize and connect to the core

• Unequal-cost paths and EtherChannel links ensure high availability, load balancing, and minimal downtime

Conclusion

EIGRP is a powerful routing protocol when designed with hierarchical structure, route summarization, and careful metric tuning. At ITVue Networks, we implement EIGRP with proper access-distribution-core design, EtherChannel, and variance-based load balancing to ensure enterprise networks are resilient, efficient, and scalable.