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New Member

OSPF cost calculation

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

Hall of Fame Super Bronze

OSPF cost calculation

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.

New Member

OSPF cost calculation

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

New Member

OSPF cost calculation

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

New Member

OSPF cost calculation

please have a look and let me know

Hall of Fame Super Bronze

OSPF cost calculation

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.

VIP Super Bronze

OSPF cost calculation

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.


Re: OSPF cost calculation

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"