People will say “Hey! What is there in OSPF Reference Bandwidth………..its 100 Mbps”.
Yes, you are correct but there are few more things which you should keep in mind.
By default, OSPF reference bandwidth is 10^8 i.e 100 Mbps but yes you can change it to any desired value but since OSPF link cost is a 16 bit number so cost can be maximum up to 65,535, hence if you change the reference bandwidth to 4294967 which is maximum in Cisco routers still the cost of your 64K link will be 65535 not 67108859 which comes according to the Metric formula. I believe Cisco did this to keep 64K link under consideration when some network is having 100G link as well as 64K link which is merely impossible.
The details of the OSPF reference bandwidth is mentioned in the RFC 4750 Page 45. Please find few details below.
Where OSPF reference bandwidth is used, it is used in Link metric calculation below is the formula:
Metric = referenceBandwidth / ifSpeed
This yields a number having the following typical values:
Network Type/bit rate Metric
>= 100 MBPS 1
T1 (ESF) 64
64 KBPS 1562
56 KBPS 1785
19.2 KBPS 5208
9.6 KBPS 10416
Ohh!!!! Now the Question is what if my interface is 1 Gbps or 10 Gbps? Whether the cost will be in fraction? Nope, it will be “1”.
In Cisco Routers you can see the reference bandwidth by the below command:
R1#show ip ospf | i band
Reference bandwidth unit is 100 mbps
You can change the reference bandwidth(to consider the high speed interfaces into ospf cost calculation) by the below command under ospf process id:
R1(config)#router ospf 50
R1(config-router)# auto-cost reference-bandwidth <value>
Then your metric calculation will happen depending on the new value. Suppose, you have changed the reference bandwidth value from default to 1000 then the cost of a T1 link will change from 64 to 647.
But, one more thing you should keep in mind that if you are assigning cost manually to any particular interface using “ip ospf cost <cost>” command then it overrides the formulated cost by the reference bandwidth.
This what I know, you are most welcome to criticize and update me my mistakes.