EIGRP Metric Calculations

🧮 EIGRP Metric Calculation Understanding the “Best Path”

Enhanced Interior Gateway Routing Protocol (EIGRP) is fundamentally a distance vector routing protocol, but it surpasses older protocols like RIP by using a sophisticated, multi-component metric system. This system determines the “best path” in the network, ensuring rapid convergence and accurate path selection across modern high-speed infrastructures.

If you’re preparing for an interview or want a deep technical revision, mastering EIGRP’s metric formula is essential — it’s the heart of the protocol’s routing logic.

📘 PART 1: THE BUILDING BLOCKS (VECTOR METRICS)

Unlike RIP, which relies solely on hop count, EIGRP uses multiple vector metrics to calculate the desirability of a route. These are accumulated from the destination back to the source.

EIGRP uses five key vector metrics:

Metric Component	Description	Technical Details
Bandwidth (BW)	Lowest configured bandwidth along the entire path	Expressed in kbps. Used as inverse minimum bandwidth, scaled by a factor of 10⁷
Delay (DLY)	Cumulative transmission delay across all outgoing interfaces	Measured in tens of microseconds (μs)
Reliability (REL)	Indicates link quality (error rate)	Value between 1–255, with 255/255 = 100% reliable
Load (LOAD)	Indicates link utilization or congestion	Range 1–255, where 255 = 100% saturation
MTU	Minimum MTU size along the path	Not used in composite metric but carried in vector

Accumulation Rules:

• Bandwidth: Minimum value across the path
• Delay: Summation of all delays
• Reliability: Minimum value
• Load: Maximum value
• MTU: Minimum value
• Hop Count: Accumulative

⚙️ PART 2: THE CLASSIC METRIC FORMULA (32-BIT)

EIGRP’s composite metric is derived from IGRP’s formula, multiplied by 256 to extend it from 24 bits to 32 bits.

📐 Full Formula:

Metric = 256 × [(K1 × BW + (K2 × BW) / (256 - Load) + K3 × Delay) × (K5 / (K4 + Reliability))]

Where:

• BW → Inverse minimum bandwidth (scaled to 10⁷)
• Delay → Total delay in tens of microseconds (μs)
• If K5 = 0, the reliability quotient is defined as 1

⚖️ Default K Values:

| Coefficient | Purpose | Default Value | |————–|———-|—————-| | K1 | Bandwidth | 1 | | K2 | Load | 0 | | K3 | Delay | 1 | | K4 | Reliability | 0 | | K5 | Reliability | 0 | | K6 | Extended Attributes (Wide Metrics only) | 0 |

Note:
All K-values must match between neighbors for adjacency formation and route exchange.

🧮 Simplified Default Formula:

When default K-values are applied:

Metric = 256 × ( (10⁷ / Minimum Bandwidth) + Total Delay )

• Bandwidth → in kilobits per second (kbps)
• Delay → in tens of microseconds (μs)

────────────────────────────────────────────────────────────────────────────── 🚀 PART 3: WHY WIDE METRICS WERE INTRODUCED ──────────────────────────────────────────────────────────────────────────────

Classic metrics had scalability issues on high-speed links (1G, 10G, etc.). For instance, both GigabitEthernet and TenGigabitEthernet could result in the same calculated metric — preventing accurate differentiation of link speeds.

Wide Metrics were introduced to address this issue by providing greater precision and scalability.

🔧 Key Enhancements in Wide Metrics:

1. Scaling Factor:
   • Classic: ×256
   • Wide: ×65,535
2. Delay Unit:
   • Classic: microseconds (10⁻⁶)
   • Wide: picoseconds (10⁻¹²) — much finer granularity
3. New Coefficient:
   • K6 introduced for Extended Attributes (e.g., Jitter, Energy metrics)

📐 Wide Metric Formula:

Wide Metric = 65,535 × [(K1 × BW + (K2 × BW) / (256 - Load) + K3 × Latency + K6 × Extended) × (K5 / (K4+ Reliability))]

────────────────────────────────────────────────────────────────────────────── 🧭 PART 4: CONFIGURATION & KEY TAKEAWAYS ──────────────────────────────────────────────────────────────────────────────

🔸 Metric Styles:

| Configuration Mode | Metric Type Used | |——————–|——————| | Classic Mode | Classic Metric | | Named Mode | Wide Metric |

🔸 Influencing EIGRP Path Selection:

You can modify EIGRP’s route decision process by tuning metric components:

1. Interface Delay
   • Adjusting delay affects only EIGRP calculations (preferred method).
   • Configured in tens of microseconds.
   • Command Example:

     Router(config-if)# delay <tens-of-microseconds>
     

2. K-Values (Metric Weights)
   • Modify the influence of bandwidth, delay, load, or reliability.
   • K-values must match between neighbors.
   • Command Example:

     Router(config-router)# metric weights 0 K1 K2 K3 K4 K5
     

3. Offset Lists
   • Add a static offset to route metrics to make a path less desirable.
   • Commonly used for manual route preference tuning.
   • Command Example:

     Router(config-router)# offset-list <access-list> in/out <offset-value> <interface>
     

────────────────────────────────────────────────────────────────────────────── 🏁 SUMMARY ──────────────────────────────────────────────────────────────────────────────

✅ EIGRP Metric Components: Bandwidth, Delay, Load, Reliability, MTU
✅ Default Calculation Uses: Bandwidth + Delay
✅ Classic Metric: 32-bit; scaled by ×256
✅ Wide Metric: 64-bit-like precision; scaled by ×65,535
✅ Key Configuration Difference: Classic → Classic Mode; Wide → Named Mode
✅ Influence Path Selection: Adjust Delay or Offset Lists
✅ Successor Route: The path with the lowest composite metric
✅ Supports: Unequal-cost load balancing using the variance command

────────────────────────────────────────────────────────────────────────────── 🎯 FINAL THOUGHT: ────────────────────────────────────────────────────────────────────────────── The EIGRP metric formula is the beating heart of EIGRP’s intelligence. It not only determines the successor and feasible successor routes but also enables loop-free convergence and load balancing in real time. Understanding it gives you complete control over how EIGRP “thinks” when choosing the best path in your network. ──────────────────────────────────────────────────────────────────────────────

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