OSPF Cost

Christopher Dobkowski

Hey there fellows

So I have this, rather in detail question. I was just reading about the OSPF and I came to a point where the cost is discussed.
Well it says that the cost is measured by dividing 10⁸ by the bandwidth of the link in bps. Ok cool...

Afterwards you give me a table with the common costs you'll find associated with their link speed. And you give me the link speed of 56 kbps which is equivalent to 57344 bps right? Ok. Next of, the 10 to the power of 8 = 100000000, riiiiight

Then you tell me the OSPF cost of a 56 kbps link is 1785, where based on math it's 1743.

Why?

okplaya

If I'm not mistaken, with storage you use kilo = 1024. With line speed kilo = 1000, or at least that what Cisco uses.

So for your question, OSPF is using 56000 and not 57344. I hope this makes sense.

Here's a cool link to check - iCalc - Bandwidth Calculator - Transfer Rate, KB, MB, GB, File Size - iBeast Business Solutions

MrBrian

Where does it say 56kbps = 57344 bps? If you do the calculation 100000000/56000 then it's 1785.

Sometimes different places have different conversions, something I'm still trying to understand. For instance 1 MB = 1,048,576 bytes.. but some places will say that 1 MB = 1,000,000 bytes straight up. I think for the purposes of this OSPF calculation, and other bandwidth related calculations, they're using even metric values i.e. 1KB = 1000 and 1GB = 1,000,000.

I think the other method is used when referring to hard drive size. For instance 1 Kilobyte = 1024 bytes (2^10). And it sounds like when it's using this format to express a number of bits, they can be called Kibibytes and Mebibytes.. which can get a "bit" confusing, ha.

Have a look at this page which explains it somewhat. Not sure of its validity, but it does coincide with information I've read in the past. Anyone else want to drop knowledge as to the difference please do..
Conversion Calculator Bit Byte Kilobyte Megabyte Gigabyte Terabyte Petabyte Exabyte

boredgamelad

The problem you're running into is a classic one. When we're talking about data rates:

Kilobit (kbit) = 1000 bits, not 1024 bits.

Therefore,

56kbit/s (or kbps) = 56,000 bit/s
1 Mbit/s (or Mbps) = 1,000,000 bit/s

Some more reading in addition to what MrBrian posted: Data rate units - Wikipedia, the free encyclopedia

Christopher Dobkowski

Oh so it's not 1024 but 1000 when you're doing the bandwidth math...

Thanks to every one