📘 CCNA 200-301 v1.1

3.2 Determine how a router makes a forwarding decision by default

3.2.c Routing protocol metric

🔹 What Is a Routing Protocol Metric?

A metric is a value used by a routing protocol to determine the best path to reach a destination network when there are multiple possible routes.

Think of the metric as a score or cost assigned to each path — the lower the metric, the better the path (for most protocols).

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🔹 Why Do We Need Metrics?

When routers learn about the same destination network from different routes (for example, from different neighbors or interfaces), the router needs a way to compare those routes.

The metric gives the router a way to choose the most efficient or preferred path.

Example in an IT context:

• Router R1 can reach the 10.1.1.0/24 network through two different routers: R2 and R3.
• Both routes are learned using the same routing protocol.
• R1 compares the metric values for each route.
• The route with the lower metric is placed in the routing table and used for forwarding traffic.

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🔹 How Routing Protocols Use Metrics

Different routing protocols calculate metrics differently.
Each protocol has its own method and factors it considers important.

Let’s go through each major routing protocol you need to know for the CCNA exam.

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1. RIP (Routing Information Protocol)

• Metric used: Hop count
• Definition: Each router hop (each router between source and destination) increases the hop count by 1.
• Path with fewer hops = better path.
• Maximum hop count: 15
  (If more than 15 hops → network is considered unreachable.)
• Example:
  • Path A = 2 hops
  • Path B = 4 hops
    → Router will choose Path A (metric 2) because it’s shorter.

🟩 Summary for RIP:

• Metric type: Hop count
• Best path: Lowest hop count
• Max hops: 15

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2. EIGRP (Enhanced Interior Gateway Routing Protocol)

• Metric used: Composite metric (based on several factors)
• Factors included:
  • Bandwidth (minimum bandwidth along the path)
  • Delay (cumulative delay of all links)
  • Reliability (optional)
  • Load (optional)
• By default, only bandwidth and delay are used in the calculation.
• Formula (simplified): Metric = (Bandwidth + Delay) × 256 (Cisco routers actually use a more complex formula, but for CCNA, know that it’s based mainly on bandwidth and delay.)

🟩 Summary for EIGRP:

• Metric type: Composite metric (uses bandwidth and delay)
• Best path: Lowest metric
• Default formula includes: Bandwidth and Delay

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3. OSPF (Open Shortest Path First)

• Metric used: Cost
• Cost is based on the bandwidth of the interface.
• Formula: Cost = Reference Bandwidth / Interface Bandwidth
• Reference Bandwidth (default) = 100 Mbps
  (This can be changed with the command auto-cost reference-bandwidth.)
• The higher the bandwidth, the lower the cost.

🟩 Example:

Interface	Bandwidth	Cost (Default)
FastEthernet (100 Mbps)	100 Mbps	1
GigabitEthernet (1 Gbps)	1000 Mbps	0.1 (rounded to 1)
Serial (1.544 Mbps)	1.544 Mbps	~64

🟩 Summary for OSPF:

• Metric type: Cost
• Formula: Reference bandwidth / interface bandwidth
• Best path: Lowest cost

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4. IS-IS (Intermediate System-to-Intermediate System)

• Metric used: Cost
• By default, all interfaces have a default cost (usually 10), but it can be manually configured.
• Works similar to OSPF (chooses path with lowest total cost).

🟩 Summary for IS-IS:

• Metric type: Cost (configurable)
• Best path: Lowest cost

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5. BGP (Border Gateway Protocol)

• Metric used: Path attributes (not a single numeric metric)
• BGP uses multiple attributes to select the best path.
• Important attributes (in order of importance for path selection):
  1. Weight (Cisco proprietary, highest preferred)
  2. Local Preference (highest preferred)
  3. Shortest AS Path (fewest autonomous systems)
  4. Origin type (IGP < EGP < Incomplete)
  5. MED (Multi-Exit Discriminator) (lower preferred)
  6. eBGP over iBGP
  7. Lowest router ID (if all else equal)

🟩 Summary for BGP:

• Metric type: Uses multiple attributes
• Best path: Determined through a series of attribute comparisons

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🔹 How Metrics Affect Routing Decisions

When a router has multiple routes to the same destination from the same routing protocol, it compares metric values and:

• Chooses the lowest metric as the best route
• Installs only that route in the routing table

If multiple routes have equal metrics, load balancing may occur (equal-cost load balancing).

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🔹 What Happens When Routes Come from Different Protocols?

When the same destination is learned from different routing protocols (e.g., OSPF and RIP), the router uses Administrative Distance (AD) first to decide which protocol to trust more.
Then, within that protocol, it uses the metric to select the best route.

🟦 Example (IT scenario):

• RIP route to 10.1.1.0/24 → AD 120, Metric 2
• OSPF route to 10.1.1.0/24 → AD 110, Metric 10
  → Router picks OSPF route because its AD is lower.
  → Then, within OSPF, it uses cost to choose the best path.

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🔹 Summary Table (CCNA Must-Know)

Routing Protocol	Metric Type	Lower = Better?	Key Metric Factors
RIP	Hop Count	✅	Number of routers in path
EIGRP	Composite	✅	Bandwidth + Delay
OSPF	Cost	✅	Bandwidth
IS-IS	Cost	✅	Configurable value
BGP	Path Attributes	Complex	Weight, Local Pref, AS Path, etc.

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🔹 Key Exam Points

✅ Know which metric each protocol uses.
✅ Know that lower metric = better path (for most IGPs).
✅ Understand how OSPF and EIGRP calculate their metrics.
✅ Remember that Administrative Distance is used before metrics when comparing routes from different protocols.
✅ Know that equal-cost routes can be load-balanced.

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🔸 In Summary:

A routing protocol metric is the value used by routers to compare multiple paths learned by the same protocol.
It allows routers to choose the most efficient path for forwarding data within an IP network.
Different protocols use different metric systems — but they all aim to find the best and most reliable path to deliver packets successfully.