OSPF cost calculation

[zaidiatifs]

hi people , i need to undertand how OSPF calculates the cost as its metric, i know the the formula to calculate is  100000000/interface bandwidth, but how can we convert the interface bandwidth which is in KBIT to BPS ?

please let me know the detailed procedure to accomplish this .

also for the serial Links i have noticed that the interface bandwidth is 1544 kbit or 128 kbit , also for the fast ethernet or ethernet interfaces the badwidth seems to be 100000 kbit

please specify the calculation of cost for these values

kindly let me know that ,

also kindly suggest me a Topology ( OSPF network diagram ) where i can get to see DR and BDR's as i believe it only happens on broadcast and NBMA

[Edison Ortiz]

Default is 100Mbps/Interface Bandwidth

56-kbps serial link—Default cost is 1785

•64-kbps serial link—Default cost is 1562

•T1 (1.544-Mbps serial link)—Default cost is 64

•E1 (2.048-Mbps serial link)—Default cost is 48

•4-Mbps Token Ring—Default cost is 25

•Ethernet—Default cost is 10

•FastEthernet—Default cost is 1

•16-Mbps Token Ring—Default cost is 6

•FDDI—Default cost is 1

•X25—Default cost is 5208

•Asynchronous—Default cost is 10,000

•ATM— Default cost is 1

You can use DR and BDR on any link topology if desired, simply change the ospf network type under the interface.

[zaidiatifs]

i already know that ..what i need to know is how these are calculated using the formula

[Sanjay Shaw]

Hi Atif,

First of all to let u know your query is not clear and next to it there is no term KBIT in Cisco.

bandwidth which is in KBIT to BPS

[zaidiatifs]

please have a look and let me know

[Edison Ortiz]

So, 100Mbps/1.544Mbps = ?

If you have a 128Kbps connection, 100Mbps/.128Mbps =?

That's why I provided the table. A quick math will given you the answer.

[Reza Sharifi]

Not sure exactly what your question is but:

8=Bits=1 Byte

1024 Byte=1 Kilobyte

1024 Kilobyte=1 Megabyte

So, in your case, the interface is 1544 Kilobyte which is equal to about 1.5 Megabyte.

and the OSPF cost is 64. as Edison already answerd that.

HTH

[Rolf Fischer]

Although the question has already been answered perfectly, I'd like to add two details:

1. The standards do not define the way how the cost is calculated; the formula above referes to the (wise) Cisco implemetation. I've heard that some vendors set the default costs of any interface to 1 (Hopcount, like RIP).
2. 100 Mbps is the default value for the numerator. That's why I prefer the formula "reference-bandwidth/link-speed"

Regards

Rolf