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What is EIGRP (Enhanced Interior Gateway Routing Protocol)?

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Enhanced Interior Gateway Routing Protocol (EIGRP) is a sophisticated and efficient routing protocol designed for use in computer networks, particularly in IP networks. EIGRP falls into the category of advanced distance vector routing protocols, but it incorporates elements of link-state protocols, making it more feature-rich and adaptable. Here's a technical breakdown of EIGRP:

  1. DUAL (Diffusing Update Algorithm):
    • EIGRP uses the Diffusing Update Algorithm (DUAL) to calculate the best routes to destination networks. DUAL is responsible for ensuring loop-free and optimal routing by dynamically adapting to changes in the network topology.
  2. Neighbor Discovery and Maintenance:
    • EIGRP routers establish and maintain neighbor relationships to exchange routing information. This process involves the exchange of Hello packets, and when routers become neighbors, they can share routing information.
  3. Topology Table:
    • EIGRP routers maintain a topology table, which holds detailed information about all known routes. This information includes metrics, successor routes, and feasible successor routes. The topology table is used by the DUAL algorithm to determine the best routes.
  4. Composite Metric Calculation:
    • EIGRP uses a composite metric to evaluate the best route to a destination. This metric is calculated using various parameters, including bandwidth, delay, load, and reliability. The K-values (constants) can be adjusted to influence the metric calculation based on network characteristics.
  5. Feasibility Condition:
    • EIGRP uses the feasibility condition to identify backup or feasible successor routes. This condition is checked during topology changes, and if met, the backup route can be used immediately without the need for a recomputation of the entire topology.
  6. Convergence:
    • EIGRP is designed to converge quickly when there are changes in the network. It achieves this by using features like DUAL and the feasibility condition, allowing for rapid adaptation to network topology changes while maintaining loop-free routing.
  7. Protocol-Dependent Modules (PDM):
    • EIGRP consists of modules known as Protocol-Dependent Modules (PDMs) that handle specific network layer protocols. These modules allow EIGRP to support multiple routed protocols, such as IPv4 and IPv6.
  8. Incremental Updates:
    • EIGRP sends incremental updates, which means that only the changes in the network state are transmitted rather than the entire routing table. This minimizes the amount of bandwidth used for routing updates.
  9. Reliability and Load Balancing:
    • EIGRP supports load balancing across equal-cost paths, distributing traffic efficiently across multiple links. Additionally, it has mechanisms to ensure the reliability of routing information and to detect and handle errors or inconsistencies in the network.
  10. Stub Routing:
    • EIGRP supports stub routing, allowing certain routers to be configured as stubs. Stub routers have limited routing information and do not participate fully in the EIGRP network, reducing the amount of routing information they need to maintain.