How OSPF metric?

Discussion in 'Cisco' started by BB, Jan 5, 2005.

  1. BB

    BB Guest

    I have a question about OSPF cost. According cisco, to calculate cost in
    OSPF, we use the formula: 10^8/BW. What's OSPF cost in Gigabit/10Gigabit
    Ethernet interfaces

    Thank you
     
    BB, Jan 5, 2005
    #1
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  2. BB

    Hansang Bae Guest

    You can use "ip ospf cost" under the interfaces to manually set the
    costs. Or you can use "auto-cost reference-bandwidth xxx' under the
    OSPF process to modify the FastE-OCX+-FDDI-GigE == 1 problem.


    hsb


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    Hansang Bae, Jan 5, 2005
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  3. BB

    Ivan Ostreš Guest

    AFAIK, cost for interfaces with bandwidth equal or larger than 10^8 bps
    is normalized to 1. It is not a good practice to left it that way
    because it will not "see" the difference between (high bandwidth)
    circuits.

    The main reason for all this is that people which created OSPF spec in
    the first place, didn't expect to have such a great bandwidth that is
    available today. Seems it was a common mistake in IT in the past (just
    remember DOS and 640K limit).

    To fix problems you could use manual 'ip ospf cost' interface command
    which will set appropriate cost to specified inteface. This might look
    as a solution to a problem, but really it is not because you need to
    lower bandwidth on Gig interfaces and you can't go into negative cost.

    Probably the best way would be to use 'auto-cost reference-bandwidth'
    command under your routing process command mode. It will put specified
    bandwidth as a reference. The important thing is that you have to put
    the same reference bandwidth to all OSPF routers in your OSPF domain to
    get desired results.

    You should be warned that while doing this, you might loose connectivity
    on your routers for a while because routers have to run SPF algoritm
    again. It could also create additional connectivity problems because of
    possible asynchronous routing while not all the 'reference-bandwidth'
    commands are in their place.

    HTH,
     
    Ivan Ostreš, Jan 5, 2005
    #3
  4. BB

    peart

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    peart, Jun 18, 2008
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