OSPF Cost Visualizer - Interactive Learning Tool

The Big Picture

OSPF needs to pick the best path to send your data. But what makes a path "best"?

Here's the thing: it's not about counting how many routers you pass through. It's about finding the fastest route overall.

Think of it Like GPS

"Your GPS doesn't just count streets. It knows the highway is faster than the back road, even if the back road has fewer turns. OSPF does the same thing with network links."

When you ask for directions, GPS considers how fast each road is, not just how many roads. A path with 5 highway segments beats a path with 2 dirt roads every time.

Cost = Speed Rating

Every network link gets a number called "cost". It's simple:

Lower number = Faster link = OSPF prefers it
Higher number = Slower link = OSPF avoids it if possible

That's really all there is to it. Fiber gets a low cost because it's fast. Old serial lines get a high cost because they're slow.

Quick Reference

Link Type	Speed	Cost	Think of it as...
10G Fiber	10 Gbps	1	Highway - wide open, full speed
1G Fiber	1 Gbps	10	Main road - good traffic flow
FastEthernet	100 Mbps	100	Side street - gets the job done
T1/Serial	1.544 Mbps	64	Dirt road - slow but sometimes necessary

The One Rule to Remember

OSPF adds up the costs along a path. Lowest total wins.

Why Should I Care?

When you understand cost, you control where traffic flows. This is called traffic engineering.

Want backups to use the cheap slow link? Raise the cost on the fast link.
Want video calls to always use fiber? Make sure fiber has the lowest cost.
Need to take a link down for maintenance? Raise its cost sky-high first, traffic moves automatically.

Want More Detail?

How is cost actually calculated?

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There's a simple formula:

Cost = Reference Bandwidth / Interface Bandwidth

By default, the reference bandwidth is 100 Mbps. So:

100 Mbps link: 100 / 100 = Cost 1
10 Mbps link: 100 / 10 = Cost 10
1 Gbps link: 100 / 1000 = 0.1 → rounds to Cost 1

Notice the problem? Both 100 Mbps and 1 Gbps get cost 1! That's why modern networks change the reference bandwidth to 10 Gbps or higher.

Can I change the cost manually?

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Absolutely! Two ways:

1. Change cost on a specific interface:

Router(config)# interface GigabitEthernet0/1
Router(config-if)# ip ospf cost 50

2. Change the reference bandwidth (affects all interfaces):

Router(config)# router ospf 1
Router(config-router)# auto-cost reference-bandwidth 10000

The 10000 means 10,000 Mbps (10 Gbps). Do this on ALL routers in your OSPF domain!

What if two paths have equal cost?

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Great question! When OSPF finds multiple paths with the exact same total cost, it uses both of them. This is called Equal-Cost Multi-Path (ECMP).

Traffic gets load-balanced across all equal-cost paths automatically. Free performance boost!

By default, OSPF can use up to 4 equal-cost paths (configurable up to 16 on most platforms).

Real-world example: Traffic engineering

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Scenario: You have two paths between offices:

Path A: Expensive dedicated fiber (fast, reliable)
Path B: Cheap internet VPN (slower, best-effort)

Goal: Use fiber for everything, but automatically fail over to VPN if fiber dies.

Solution:

Set fiber interface cost to 10
Set VPN interface cost to 1000

OSPF will always prefer the fiber (lower cost). If fiber fails, it automatically switches to VPN. When fiber comes back, traffic shifts back. No manual intervention needed!

Ready to see it in action? Try the interactive tabs above! →

Click any router to see its OSPF view of the network

Link Types

10G Fiber (Cost 1)

1G Fiber (Cost 10)

FastEthernet (Cost 100)

Serial/T1 (Cost 64)

Click a router to view its
SPF tree and routing table

Click a source router, then a destination router to explore paths

Link Types

10G Fiber (Cost 1)

1G Fiber (Cost 10)

FastEthernet (Cost 100)

Serial/T1 (Cost 64)

Select source and destination
to compare available paths

Click any link to fail it, or click a router to see routing changes

Simulation Controls

Link Types

10G Fiber (Cost 1)

1G Fiber (Cost 10)

FastEthernet (Cost 100)

Serial/T1 (Cost 64)

Fail links to see how OSPF
reconverges automatically