Networking - EIGRP (Enhanced Interior Gateway Routing Protocol)

Enhanced Interior Gateway Routing Protocol (EIGRP) is an advanced distance-vector routing protocol developed by Cisco Systems. It combines the best features of both distance-vector and link-state routing protocols, making it a hybrid routing protocol.

EIGRP provides fast convergence, efficient use of bandwidth, and high scalability, which makes it suitable for large enterprise networks. It was originally a Cisco proprietary protocol, but later became an open standard (as defined in RFC 7868).

1. Definition

EIGRP is a dynamic routing protocol that automatically shares routing information between routers in the same autonomous system (AS). It uses the Diffusing Update Algorithm (DUAL) to ensure loop-free and efficient route computation, providing quick convergence and reliable network operation.

In simple terms:

EIGRP dynamically finds the best path to each network and adapts quickly to topology changes, ensuring data always follows the most efficient route.

2. Characteristics of EIGRP

3. How EIGRP Works

EIGRP operates by forming neighbor relationships with other EIGRP routers, exchanging routing information, and maintaining a topology table to select the best and backup routes.

Step-by-Step Operation:

1. Neighbor Discovery:

   • Routers send Hello packets to discover and establish adjacency with other EIGRP routers on directly connected networks.

   • Once routers agree on parameters (AS number, K-values, etc.), they become neighbors.

2. Exchange of Routing Information:

   • Neighbors exchange EIGRP Update packets containing route information.

   • Each router builds its topology table, which holds all learned routes.

3. DUAL Calculation:

   • Using the Diffusing Update Algorithm (DUAL), EIGRP selects the best path to each destination and a feasible successor (backup path).

4. Route Installation:

   • The best path (successor) is installed in the routing table.

   • If the primary path fails, the feasible successor provides immediate failover (fast convergence).

5. Periodic Updates:

   • Unlike RIP, EIGRP does not send periodic updates. It sends updates only when topology changes, minimizing bandwidth usage.

4. EIGRP Tables

EIGRP maintains three main tables:

5. EIGRP Packet Types

EIGRP uses five packet types for communication between routers:

6. EIGRP Metric Calculation

EIGRP uses a composite metric that considers multiple parameters such as bandwidth, delay, reliability, load, and MTU.

By default, only bandwidth and delay are used to calculate the metric.

[
\text{Metric} = 256 \times \left( \frac{10^7}{\text{Minimum Bandwidth (in Kbps)}} + \text{Cumulative Delay (in 10 µs)} \right)
]

EIGRP uses K-values (K1–K5) to enable or disable certain parameters in the metric calculation.

7. DUAL (Diffusing Update Algorithm)

DUAL is the heart of EIGRP — it ensures fast convergence and loop-free routing.

• Successor: The best route to a destination (lowest metric).

• Feasible Successor (FS): A backup route that meets the feasibility condition.

• Feasibility Condition (FC):
  A neighbor’s advertised distance (AD) must be less than the router’s feasible distance (FD) to be considered a feasible successor.

If the successor fails, the router immediately switches to the feasible successor without recalculating — ensuring near-instant convergence.

8. EIGRP Metrics Explained

9. EIGRP Features

• Partial and Bounded Updates:
  Sends updates only when needed and only to affected routers.

• VLSM and CIDR Support:
  Fully supports variable-length subnet masks and classless routing.

• Load Balancing:
  Supports Equal-Cost and Unequal-Cost Load Balancing (with the “variance” command).

• Fast Convergence:
  DUAL ensures instant route recovery using feasible successors.

• Authentication Support:
  Can authenticate routing updates using MD5 or SHA.

• Protocol Independence (PDM):
  EIGRP supports multiple network protocols such as IPv4, IPv6, IPX, and AppleTalk through Protocol-Dependent Modules (PDMs).

10. EIGRP Neighbor Relationships

To form adjacency, routers must match:

• Autonomous System (AS) number

• K-values (metric weights)

• Subnet and Hello/hold timers

• Authentication (if configured)

Hello packets are sent every 5 seconds (LAN) or 60 seconds (WAN).
If no Hello packet is received within the Hold Time, the neighbor is considered down.

11. Advantages of EIGRP

1. Fast Convergence:
   DUAL allows for near-instant recovery from topology changes.

2. Efficient Bandwidth Use:
   Sends partial updates only when necessary.

3. Scalable:
   Performs well in both small and large networks.

4. Supports VLSM and CIDR:
   Makes IP address usage more efficient.

5. Unequal-Cost Load Balancing:
   Can distribute traffic across multiple paths with different metrics.

6. Loop-Free Operation:
   DUAL ensures loop prevention through feasibility conditions.

12. Disadvantages of EIGRP

1. Originally Cisco Proprietary:
   Though now open, full multi-vendor support can vary.

2. Complex Configuration:
   More advanced than RIP and requires deeper understanding.

3. Limited Use in ISPs:
   Primarily used in enterprise networks, not in Internet backbone routing.

4. Resource Intensive:
   Requires more CPU and memory than simpler protocols like RIP.

13. EIGRP vs OSPF

14. Real-World Applications

• Enterprise Networks: Core routing within Cisco-based organizations.

• Campus and Branch Networks: Reliable and efficient routing across distributed sites.

• Hybrid Networks: Environments that use both IPv4 and IPv6 routing.

• Large Corporations: Where fast convergence and scalability are critical.

15. Conclusion

Enhanced Interior Gateway Routing Protocol (EIGRP) is one of the most powerful and efficient interior routing protocols in use today. Combining the best of distance-vector and link-state mechanisms, EIGRP offers fast convergence, loop-free routing, low overhead, and support for unequal-cost load balancing.

Although originally Cisco-proprietary, its performance, reliability, and scalability make it a preferred choice in enterprise-grade networks where speed and stability are paramount.