EIGRP Feasibility Condition

🧭 EIGRP Feasibility Condition (FC): The Secret Behind Loop-Free Routing

EIGRP (Enhanced Interior Gateway Routing Protocol) is famous for its lightning-fast convergence — recovering from network failures faster than most other IGPs. The magic behind this speed is the Feasibility Condition (FC) — a core concept derived from EIGRP’s Diffusing Update Algorithm (DUAL).

The FC ensures loop-free paths, pre-computed backups, and near-instant convergence when primary links fail.

🔹 Part 1: Understanding the DUAL Terminology

Before diving into the Feasibility Condition, let’s define EIGRP’s fundamental components — all stored in the Topology Table.

Term	Easy Definition	Technical Definition
Feasible Distance (FD)	The best total metric currently known to reach a destination.	The lowest known total metric to a destination since the last ACTIVE → PASSIVE transition.
Reported Distance (RD)	The metric advertised by a neighbor toward the destination.	The neighbor’s advertised distance to reach that network, shared in routing updates.
Successor	The best path used for actual traffic forwarding.	The neighbor that provides the lowest-cost path and meets the Feasibility Condition.
Feasible Successor (FS)	A pre-computed backup route that’s guaranteed loop-free.	A backup route that satisfies the Feasibility Condition (RD < FD).

🔹 Part 2: The Feasibility Condition Rule

The Feasibility Condition decides whether a potential route is loop-free and eligible to become a Feasible Successor.

✅ The Golden Rule:

            Reported Distance (RD)<Feasible Distance (FD)

If this condition holds true, the neighbor’s path is guaranteed loop-free and stored as a Feasible Successor.

🧩 Why RD < FD Prevents Loops

Let’s visualize:

Destination Network (10.1.1.0/24)
        |
      [R3]
       | RD = 100
      [R2]
       | RD = 200 <--- R1’s FD = 300
      [R1]

Explanation:

• R1’s best known total cost (FD) to the destination = 300
• R2 advertises its distance (RD) = 200
• Since RD (200) < FD (300), R2 is closer to the destination than R1
• Hence, R2 cannot route traffic back to R1 — ensuring a loop-free path

If instead:

RD>FD

That means the neighbor (R2) might be upstream and could potentially send traffic back toward R1, forming a routing loop.

Thus, EIGRP never considers such a path as a valid backup.

EIGRP’s DUAL (Diffusing Update Algorithm) maintains three key data structures:

+---------------------------------------+
|             EIGRP Tables              |
+---------------------------------------+
| Neighbor Table   → List of neighbors  |
| Topology Table   → All known routes   |
| Routing Table    → Best route only    |
+---------------------------------------+

• Successor → Best path (active route in RIB)
• Feasible Successor → Backup route (pre-validated by FC)

DUAL constantly checks FC and precomputes backup paths — allowing instant failover if the primary link fails.

🔹 Part 4: Practical Benefits of the Feasibility Condition

1️⃣ Fast Convergence

If the primary route fails, EIGRP checks if a Feasible Successor exists.

• If FS exists:
  • Instant switchover — no queries or route recalculations. Route stays in PASSIVE state.
• If FS doesn’t exist:
  • Route becomes ACTIVE → router sends Queries to neighbors → waits for replies → slower convergence.

+-------------------+       +-------------------+
| Primary Route Down| --->  | FS Exists?        |
+-------------------+       +-------------------+
           |                          |
        YES ↓                      NO ↓
  Instantly Switch          Send Queries → Active State

2️⃣ Unequal-Cost Load Balancing

Unlike OSPF or RIP, EIGRP supports unequal-cost load balancing using the variance command.

If the Feasible Successor’s FD is within the variance multiplier of the successor’s FD, it can also be installed in the routing table:

                     FS FD≤Successor FD×Variance

This allows EIGRP to load-balance across multiple paths of different costs — all guaranteed to be loop-free because of the Feasibility Condition.

Example:

Successor FD = 1000
Variance = 2
Feasible Successor FD = 1800  → (1800 ≤ 1000×2) ✅ Eligible for Load Balancing

🔹 Part 5: Summary and Key Takeaways

Concept	Description
Feasibility Condition (RD < FD)	Ensures all backup routes are loop-free
Successor	Primary path to destination
Feasible Successor (FS)	Pre-verified loop-free backup
DUAL Algorithm	Core logic maintaining stability and fast convergence
No Queries if FS Exists	Enables instant failover
Supports Unequal-Cost Load Balancing	Adds efficiency + redundancy

🧠 Interview & Exam Quick Facts

• Formula: RD < FD
• Purpose: Loop prevention in EIGRP
• Stored In: Topology Table
• Active vs Passive:
  • Passive → Stable, converged state
  • Active → Searching for new path
• Variance Command: Enables unequal-cost load balancing
• Feasible Successor: Pre-calculated backup route
• If FC Fails: Route not used (potential loop risk)

💬 Analogy: The Downstream Neighbor Concept

Think of routers as delivery partners:

• R1 = You (dispatch center)
• R2 = Neighbor (closer delivery hub)
• Destination = Customer location

If R2 says: “I’m closer to the customer than you are,” (RD < FD), you can safely hand the package to R2 — it won’t come back.

If R2 says it’s farther, you don’t trust it — it might just route the package back to you (loop).

🧩 Final Thoughts

The Feasibility Condition (RD < FD) is the backbone of EIGRP’s reliability — it’s what gives DUAL its speed, loop-free nature, and scalability.

By mastering FC, you understand not just why EIGRP converges fast, but how it guarantees stability — even in the most dynamic networks.

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