Metric – Concept Checklist

• ✅ Metric = numerical value used by routing protocols
• ✅ Determines the “cost” of reaching a destination
• ✅ Lower metric = preferred route
• ✅ Each routing protocol calculates metric differently
• ✅ Metrics are only comparable within the same protocol
• ✅ Can be manually configured for influence and control
• ✅ Key role in route selection, load balancing, and redistribution

📘 Definition of Metric in Routing

A metric is a value assigned by routing protocols to each route, which determines the preference or desirability of a path to a destination network. The lower the metric, the more preferred the route.

🎯 Purpose of Metric in Route Selection

Routing protocols often learn multiple routes to the same destination. The purpose of the metric is to provide a quantitative basis to select the best path. It enables routers to:

• Prefer faster, more reliable, or more efficient routes.
• Support policy-based routing or load balancing.
• Make dynamic adjustments to changing network conditions.

⚖️ Difference Between Metric and Administrative Distance (AD)

📌 Example: If RIP and OSPF both know how to reach 10.0.0.0/24, AD will decide. If two OSPF paths exist, metric decides.

📡 How Routers Use Metrics to Choose the Best Path

When a router has multiple routes to a network from the same protocol, it:

• Compares the metric values of all routes.
• Selects the route with the lowest metric.
• If multiple routes have equal metric, it may install all (ECMP).
• If from different protocols, AD is used first, then metric.

📐 Metric Calculation Methods in Different Routing Protocols

1. RIP – Hop Count

• Metric = Number of routers (hops) between source and destination.
• Max = 15 hops (16 = unreachable).
• ❌ Doesn’t consider bandwidth or delay.

2. OSPF – Cost

• Metric = Reference Bandwidth ÷ Interface Bandwidth
• Default Reference = 100 Mbps
• Lower bandwidth = higher cost.

3. EIGRP – Composite Metric

• Uses formula involving bandwidth, delay, reliability, load
• Metric = [ (10⁷ / bandwidth) + delay ] × 256

4. BGP – Path Attributes

• Uses attributes: AS_PATH, LOCAL_PREF, MED, Origin, Weight
• Very policy-driven

📊 Default Metric Values and Their Meaning

🛠️ Configuring and Modifying Metrics Manually

interface GigabitEthernet0/1
 ip ospf cost 5

router rip
 network 192.168.1.0
 distance 120
 metric 3

⚖️ Equal-Cost Multi-Path (ECMP) Routing and Metrics

If a protocol finds multiple paths to the same destination with equal metrics, it may install all in the routing table.

📏 Metric Comparison Within the Same Routing Protocol Only

Metrics are not comparable across protocols.

⚙️ Metric Manipulation for Load Balancing or Policy Routing

Adjust metrics to avoid certain paths or control routing behavior.

🧭 Using Route Maps to Influence Metrics

route-map SET_METRIC permit 10
 match ip address 1
 set metric 20

router rip
 redistribute ospf 1 route-map SET_METRIC

🔁 Metric Conversions During Route Redistribution

router eigrp 1
 redistribute ospf 1 metric 10000 100 255 1 1500

🕒 Metric’s Impact on Routing Convergence Time

Lower metrics converge faster, but unstable metrics may cause flapping.

🔍 Viewing and Troubleshooting Metrics

🧠 Best Practices for Metric Design and Optimization

• ✅ Ensure consistent metric assignment
• ✅ Use manual tuning to prioritize critical paths
• ✅ Avoid route flapping by avoiding dynamic metric fluctuation
• ✅ Plan for ECMP where applicable
• ✅ When redistributing, explicitly define metrics
• ✅ Use route maps and policy routing carefully
• ✅ Document metric policies for maintenance and audits

🧾 Summary Table: Protocols & Metrics

Routing Metric Quiz

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