Metrics

1.9 Troubleshoot EIGRP (classic and named mode; VRF and global)

📘CCNP Enterprise – ENARSI (300-410)

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EIGRP (Enhanced Interior Gateway Routing Protocol) uses metrics to decide the best path to reach a destination network. Think of metrics as a score that tells EIGRP which route is “better” or “faster” for sending data.

EIGRP metrics are calculated automatically, but they can also be influenced by network admins if needed. Understanding metrics is critical for the ENARSI 300-410 exam because it determines route selection and troubleshooting.

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1. How EIGRP Metrics Work

EIGRP metrics are based on several key parameters:

1. Bandwidth (KB/s) – The speed of the slowest link along the path.
2. Delay (microseconds) – How long it takes for data to travel along the path.
3. Reliability – How stable the link is (from 0 to 255, where 255 is 100% reliable).
4. Load – How busy the link is (0 to 255, where 255 is fully used).
5. MTU (Maximum Transmission Unit) – Size of the largest packet that can travel over the link (used only for path validation, not for metric calculation).

Important: By default, EIGRP uses only bandwidth and delay to calculate its metric.

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2. EIGRP Metric Formula

The EIGRP metric is calculated using this simplified formula:$$Metric = \left[ K_1 \cdot \text{Bandwidth} + K_3 \cdot \text{Delay} \right] \times 256$$Metric=[K1​⋅Bandwidth+K3​⋅Delay]×256

Where:

• K1 and K3 are default coefficients (K1 = 1, K3 = 1)
• Bandwidth = 10^7 / (minimum bandwidth along the path in Kbps)
• Delay = sum of delays of all links along the path (in tens of microseconds)

Tip for the exam: Unless specified, always assume K1=K3=1, and K2, K4, K5 = 0.

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3. Successor vs Feasible Successor

• Successor – The best path to a network (lowest metric).
• Feasible Successor – A backup path that satisfies the feasibility condition (its reported distance < current feasible distance).

EIGRP uses metrics to determine both primary and backup routes. This ensures fast convergence if the primary path fails.

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4. Metric Components in a Network Environment

Let’s translate metrics to IT network terms instead of abstract concepts:

1. Bandwidth (slowest link)
   • Example: If a path includes a 1 Gbps switch port and a 100 Mbps WAN link, EIGRP considers the 100 Mbps link for metric calculation.
2. Delay (time taken for packets)
   • Example: High latency links like satellite WAN connections increase the delay, so EIGRP prefers paths with lower delay.
3. Reliability
   • Example: If a link frequently flaps (goes up and down), EIGRP will assign it a lower reliability score, increasing the metric.
4. Load
   • Example: A link at 80% utilization has a higher metric than an idle link.
5. MTU
   • Ensures large packets don’t get dropped, but does not affect metric directly.

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5. Metric Troubleshooting Tips (Exam Focus)

1. Show commands to check metrics:
   • show ip eigrp topology – Displays feasible distance (FD) and reported distance (RD)
   • show ip route eigrp – Shows which path EIGRP chose (successor)
2. Why a path is not chosen:
   • Lower bandwidth or higher delay can make a path less preferred
   • Link flaps reduce reliability, increasing metric
   • If K-values mismatch between routers, metric calculations differ → EIGRP won’t form neighbor relationships
3. Named Mode and VRFs:
   • Metrics work the same in named mode and per VRF, but calculations are per address family (IPv4 or IPv6).

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6. Exam Tips: What You Must Remember

1. EIGRP metrics are composite (Bandwidth + Delay by default).
2. Metric is used to select successor and feasible successors.
3. Reliability, Load, MTU are optional; only matter if K-values are changed.
4. Lower metric = preferred path.
5. Mismatched K-values between neighbors can break routing.
6. Commands to check metrics:
   • show ip eigrp topology → FD and RD
   • show ip eigrp neighbors → neighbor status
   • show ip route eigrp → selected path

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7. Quick Visual Guide (Optional for Website)

Metric Component	Default Role	Exam Tip
Bandwidth	Main factor	Min bandwidth along path matters
Delay	Main factor	Sum of delays along path
Reliability	Optional	Only affects metric if K4/K5 used
Load	Optional	High load increases metric
MTU	Optional	Checks packet size, not metric

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✅ Summary:

• EIGRP uses a composite metric based on bandwidth and delay by default.
• Lower metric → better path.
• Metrics determine successor and feasible successor routes.
• Optional components (load, reliability, MTU) matter only if K-values are changed.
• Troubleshoot metrics using show ip eigrp topology and show ip route eigrp.