EIGRP in Depth: Topology Table, DUAL, and Feasible Successors

April 2012   |   8 min read

Enhanced Interior Gateway Routing Protocol (EIGRP) is a Cisco-proprietary hybrid routing protocol combining the best of link-state and distance-vector characteristics. With faster convergence and support for unequal-cost load balancing, EIGRP remains a popular choice in Cisco-heavy environments.

Core EIGRP Concepts

EIGRP maintains three essential tables:

• Neighbor Table: Tracks directly connected EIGRP peers discovered via Hello packets.
• Topology Table: Stores all learned routes, including their metrics and feasibility status.
• Routing Table: The result of best-path selection from the topology table.

DUAL Algorithm

Diffusing Update Algorithm (DUAL) is EIGRP’s brain for loop-free route computation. It ensures rapid convergence and route recalculation through the use of queries and replies across the topology.

Key terms:

• Successor: The best route to a destination (installed in the routing table).
• Feasible Successor: A backup route satisfying the Feasibility Condition.
• Reported Distance (RD): The metric as reported by a neighbor for a given route.
• Feasible Distance (FD): The lowest known metric to a destination from the local router.

Feasibility Condition

A route is a feasible successor if RD < FD. This guarantees a loop-free alternate path. If no feasible successor exists and the successor fails, DUAL initiates a query process to discover new valid routes.

Unequal Cost Load Balancing

Using the variance command, EIGRP supports unequal-cost load balancing. This can help optimize path selection and bandwidth usage when links vary in capacity and delay.

Example:

Router(config)# router eigrp 100
Router(config-router)# variance 2
  

Authentication and Route Filtering

To improve security and control route propagation, use MD5 authentication and distribute-list or route-map filtering.

Use Cases and Scalability

EIGRP is ideal in medium to large-scale Cisco-only networks. Its scalability is improved through proper summarization and tuned timers. Stub routing further optimizes large hub-and-spoke topologies by minimizing query domains.

Summary

EIGRP offers rapid convergence, loop-free paths via DUAL, and flexible load balancing. Mastering EIGRP’s topology mechanisms and feasibility concepts is key for designing resilient, scalable routed networks.

Eduardo Wnorowski is a network infrastructure consultant and technologist.
With over 17 years of experience in IT and consulting, he brings deep expertise in networking, security, infrastructure, and transformation.
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