OSPF Metric
📘 OSPF Cost – Complete Technical Guide for Interview & Learning
Purpose: This guide is designed for learners, professionals, and interviewers who want a complete and technical understanding of how OSPF (Open Shortest Path First) uses Cost as its routing metric. It includes all important concepts, formulas, configurations, and commands in a structured format for learning, revision, and reference.
🧠 1. What is OSPF Cost?
Definition: OSPF Cost is the metric used by OSPF to determine the best (shortest) path to a destination network. Lower cost = preferred path.
Key Points:
Every OSPF-enabled interface has an associated cost.
OSPF uses cumulative cost: the total cost of all outgoing interfaces in a path.
Cost and Metric are used interchangeably (Cost is more commonly used in OSPF).
Example: If a path traverses three links with costs 1, 1, and 2, the total OSPF cost = 1 + 1 + 2 = 4.
📡 2. How OSPF Cost is Calculated
Formula:
1
OSPF Cost = Reference Bandwidth / Interface Bandwidth
🔢 Default Reference Bandwidth: 100 Mbps(100,000,000 bps)
| Interface Bandwidth | OSPF Cost (Default) |
|---|---|
| 10 Mbps | 100 / 10 = 10 |
| 100 Mbps | 100 / 100 = 1 |
| 1 Gbps | 100 / 1000 = 0.1 → Rounded up to 1 |
| 10 Gbps | 100 / 10000 = 0.01 → Rounded up to 1 |
⚠️ Important: OSPF cost values are always integers. Any value < 1 is rounded up to 1.
⚠️ 3. Suboptimal Routing with Default Costs
With the default 100 Mbps reference bandwidth:
1 Gbps and 10 Gbps links get same cost as 100 Mbps: 1
Leads to suboptimal path selection in modern networks
Equal-Cost Multi-Path (ECMP) issues: OSPF may balance across unequal links
🔄 ECMP Behavior:
OSPF supports up to 4 equal-cost paths by default
If unequal links have the same OSPF cost (e.g., 100 Mbps and 10 Gbps), traffic is unevenly balanced.
🛠️ 4. Manipulating OSPF Cost (Traffic Engineering Techniques)
🔧 Method 1: Change Reference Bandwidth (Recommended)
Use this to make OSPF cost calculations more accurate for high-speed links.
1
R1(config-router)# auto-cost reference-bandwidth 10000
This sets the reference bandwidth to 10 Gbps.
Best Practice:
Set same reference bandwidth on all routers in the OSPF domain
Choose a value equal to or greater than the fastest link
⚙️ Method 2: Manually Set OSPF Cost on Interface
Override cost calculation manually.
Command:
1
R1(config-if)# ip ospf cost 50
Use this when:
You want to influence OSPF route selection manually
Link speed does not reflect actual link desirability
🧮 Method 3: Change Interface Bandwidth Value
Affects the cost indirectly via the formula.
Command:
1
R1(config-if)# bandwidth 100000
Note:
Bandwidth is set in Kbps
Changing bandwidth affects QoS and other features
Does not change actual physical speed
👀 5. Viewing OSPF Cost – Key Show Commands
| Command | Purpose |
|---|---|
show ip ospf interface brief | Quick overview: interface cost, state, area, neighbor count |
show ip ospf interface [type/number] | Detailed interface OSPF data, including exact cost |
show ip ospf database router [router-id] | Displays LSA Type 1 data and associated metrics in the LSDB |
🧬 6. Advanced: OSPF Two-Part Metric (RFC 8042)
What: OSPF extension allowing a two-part metric in special environments
Structure:
Router-to-Network Cost (outbound)
Network-to-Router Cost (inbound)
Use Case:
Helps in multi-access/broadcast environments (e.g., satellite links)
Reduces LSA update frequency
Advertised via Extended Link TLVs with Sub-TLVs in OSPFv2
Implementation Requirement:
- All routers in the area must support this extension
📝 Summary & Key Takeaways
| Concept | Description |
|---|---|
| OSPF Cost | Metric for selecting shortest path (lower = better) |
| Default Formula | Reference Bandwidth (100 Mbps) / Interface Bandwidth |
| Default Limitation | Fails to differentiate between fast links (≥ 100 Mbps) |
| Traffic Engineering | Use methods like changing reference bandwidth, manual cost setting, or bandwidth adjustment |
| Advanced Topics | Two-part metrics for complex environments |
| Show Commands | Essential for troubleshooting and verification |
🎯 Pro Interview/Revision Tips
✅ Memorize the default formula and common OSPF costs
✅ Understand why and how to manipulate OSPF costs
✅ Be ready to explain ECMP behavior and its drawbacks
✅ Know commands to verify and troubleshoot OSPF cost
✅ Stay updated on advanced OSPF enhancements like RFC 8042
🧩 Understanding OSPF Cost is critical for designing efficient networks and performing accurate traffic engineering. Misconfigured costs can lead to routing loops, suboptimal paths, and poor bandwidth utilization.
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