4.5 EIGRP Explained

Key Concepts of EIGRP

• EIGRP Basics
• EIGRP Terminology
• EIGRP Operation
• EIGRP Metrics
• EIGRP Configuration

1. EIGRP Basics

Enhanced Interior Gateway Routing Protocol (EIGRP) is an advanced distance-vector routing protocol developed by Cisco Systems. It is designed to be efficient and fast, combining the best features of both distance-vector and link-state protocols. EIGRP is widely used in enterprise networks for its scalability and convergence speed.

Example: Think of EIGRP as a sophisticated GPS system that not only provides directions but also adapts to real-time traffic conditions, ensuring the fastest route to your destination.

2. EIGRP Terminology

EIGRP uses several key terms to describe its operation:

• Neighbor Table: A database that lists all directly connected EIGRP neighbors.
• Topology Table: Contains all known routes to destination networks, including feasible successors.
• Feasible Distance (FD): The best metric to reach a destination network.
• Reported Distance (RD): The metric reported by a neighbor to reach a destination network.
• Successor: The best route to a destination network, used for packet forwarding.
• Feasible Successor: A backup route that meets the feasibility condition.

Example: Consider the neighbor table as a list of nearby friends, the topology table as a map of all known places, and feasible successors as alternative routes in case the main route is unavailable.

3. EIGRP Operation

EIGRP operates by exchanging routing information with neighboring routers. It uses reliable multicast to send and receive updates, queries, and acknowledgments. EIGRP routers build and maintain a topology table, which is used to select the best route (successor) and backup routes (feasible successors).

Example: Imagine EIGRP as a team of scouts who regularly communicate with each other to gather information about the best paths through a forest. They always have a backup plan in case the main path is blocked.

4. EIGRP Metrics

EIGRP uses a composite metric to determine the best path to a destination network. The metric is calculated based on bandwidth, delay, reliability, and load. By default, EIGRP uses only bandwidth and delay, but other factors can be included if needed.

Example: Think of the EIGRP metric as a scorecard that evaluates different routes based on factors like speed (bandwidth), travel time (delay), reliability (road conditions), and traffic (load). The route with the highest score is chosen as the best path.

5. EIGRP Configuration

Configuring EIGRP involves enabling the protocol on the router, defining the autonomous system (AS) number, and specifying the networks to be advertised. Basic commands include:

• router eigrp [AS_number]: Enables EIGRP on the router.
• network [network_address] [wildcard_mask]: Specifies the networks to be advertised.
• passive-interface [interface]: Prevents EIGRP from sending updates on a specific interface.

Example: Configuring EIGRP is like setting up a navigation system in a car. You need to turn on the system, input your destination, and specify any restrictions on the route, such as avoiding certain roads.