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(4) EIGRP Explained | Step by Step

Detailed Study Guide on EIGRP (Enhanced Interior Gateway Routing Protocol)

Overview:

  • EIGRP is a Cisco proprietary hybrid routing protocol, meaning it incorporates both distance vector and link-state features, designed for efficient routing within an autonomous system.

  • It is categorized as an Interior Gateway Protocol (IGP) and is widely utilized in enterprise networks.

  • Key functionalities include rapid convergence, efficient usage of bandwidth, and support for multiple network layer protocols.

Key Features:

  • Rapid Convergence: EIGRP's rapid convergence is facilitated by the use of the DUAL (Diffusing Update Algorithm), allowing it to quickly recompute routes in response to changes, often within seconds. This minimizes downtime and enhances network reliability.

  • DUAL Algorithm: DUAL ensures loop-free operations through the use of feasible successors (backup routes) and successors (primary routes). When the primary route fails, EIGRP can switch to the feasible successor within milliseconds.

  • Supports Multiple Network Layer Protocols: EIGRP is primarily used with IP, but also supports IPX and AppleTalk, making it adaptable to legacy systems.

Components:

  • EIGRP Neighbors: Routers must identify neighboring routers and establish adjacencies to share routing information effectively.

    • Neighbor discovery is accomplished through Hello Packets.

  • Hello Packets: Sent every 5 seconds on LANs (or every 60 seconds on WANs), these packets are essential for forming and maintaining neighbor relationships. They include parameters like K-values and the Hold Timer, which helps in determining the lifespan of the adjacency.

  • Update Packets: Contain routing information and update neighbors about reachable networks. Unlike distance vector protocols, EIGRP only sends updates about changed routes rather than sending the entire routing table.

  • Acknowledgment Packets: Acknowledge the receipt of updates to ensure reliable delivery. They are essential for maintaining the integrity and reliability of the routing information.

  • Query Packets: Sent when a route is no longer reachable; they probe neighboring routers for alternative routes to maintain connectivity.

EIGRP Metrics:

  • Bandwidth: EIGRP uses the lowest bandwidth on the link to determine a metric. The formula for EIGRP metric calculation is:

    Metric = (K1 * Delay + K2 * Bandwidth + K3 * Load + K4 * Reliability + K5 * MTU) / (K6)

  • Delay: Represents the cumulative delay of all routers along the path to a destination and is measured in microseconds.

  • Reliability: Expressed as a value from 0 to 255, where 255 indicates high reliability. By default, the reliability is calculated based on the protocol’s internal monitoring of link stability.

  • Load: Indicates the amount of traffic currently transmitted on a link. Higher loads yield higher metric values, discouraging EIGRP from utilizing overloaded paths.

  • MTU: Determines the maximum transmission unit size and can impact the performance of routing if misconfigured between routers.

Configuration Basics:

Steps to Configure EIGRP:

  1. Enable EIGRP on Routers:Use the command:router eigrp [autonomous-system-number]where the autonomous system number must be the same for all routers you wish to form EIGRP adjacencies.

  2. Network Command:Specifies the networks participating in EIGRP. This command utilizes a network address and a wildcard mask:network [network-address] [wildcard-mask] For example, to include 192.168.1.0/24:network 192.168.1.0 0.0.0.255

  3. Configure Passive Interface (Optional):To prevent sending EIGRP hello packets on interfaces such as WAN links or IP addresses not running EIGRP:passive-interface [interface-type interface-number]

  4. Configure Metrics (Optional):Bypass default behaviors and specify metric weights according to network needs using the command:metric weights 0 K1 K2 K3 K4 K5

Advantages:

  • Efficient Bandwidth Usage: Through partial and bounded updates, it minimizes overhead and efficiently handles bandwidth during routing updates.

  • Support for VLSM and CIDR: Enables efficient subnetting and route aggregation to conserve address space without compromising flexibility.

  • Scalability: EIGRP can easily accommodate growing networks with numerous routes while maintaining performance.

Disadvantages:

  • Cisco Proprietary: Its use is primarily within Cisco environments, limiting effectiveness in mixed-vendor situations.

  • Complexity: Compared to simpler distance vector protocols like RIP, EIGRP requires more sophisticated configuration and management.

EIGRP Configuration Example:

router eigrp 10
 network 192.168.1.0 0.0.0.255
 passive-interface g0/1 # Prevents hello packets on this interface

Troubleshooting Tips:

  • Use commands like show ip eigrp neighbors to confirm adjacency status.

  • Use debug eigrp packets for monitoring real-time traffic to diagnose issues.

  • Check routing table using show ip route eigrp for EIGRP routes.

  • Ensure configurations are consistent across routers within the same EIGRP Autonomous System (AS).

Conclusion:

EIGRP is a versatile and efficient routing protocol that offers significant advantages for managing dynamic routing in enterprise networks. Understanding its components, configuration, and unique features is essential for network engineers and administrators striving for optimal performance.