Metric Calculation Formulas

Question 1

How is the Cost of a Link calculated in OSPF?

Answer 1

• Reference Bandwidth / Interface Bandwith
• Default reference bandwidth is 100 MB
• Example 1 using serial link running at 1Mbps :
  
  • 100 / 1 MB serial link = cost of 100
• Example 2 using serial link running at 1.54 Mbps
  
  • 100 / 1.54 Mbps serial link = cost of 64.935 (round down to 64)

Question 2

What is the formula for calculating a metric in EIGRP?

Answer 2

• (( 10⁷ / least-bandwidth in kbps) + cumulative delay in tens of microseconds ) * 256
• Example using 10Mbps least bandwith and cumulative delay of 20 milliseconds:
  
  • 1 Mb = 1000 kb so 10 Mb = 10,000 kb
  • 1 millisecond = 1000 microseconds so 20 milliseconds = 20,000 microseconds
  • So, formula would be
  • (( 10⁷ / 10,000 kbps) + 20,000) * 256
  • = (1,000 + 20,000) * 256
  • = 21,000 * 256 = 5,376,000
  • Metric = 5,376,000

Question 3

Memorize EIGRP formula.

Answer 3

(( 10⁷ / least_bandwidth) + Delay) * 256)

Question 4

How is Delay calculated in an EIGRP metric?

Answer 4

• Delay in the formula means Serialization Delay of the interfaces.
• ‘show int s0/0’ will show a parameter Dly. That’s serialization delay.
• To find Cumulative Delay add the Dly numbers up for each outgoing interface along the path.