Quick question about EIGRP & OSPF

Discussion in 'Cisco' started by silentbob, Aug 8, 2006.

  1. silentbob

    silentbob Guest

    I'm doing some studying for my CCNP and amd trying to remember
    something I read a while back about how OSPF and EIGRP calculate their
    routes differently. It went something like:

    "The metric of one protocol is determined by the slowest link in a
    route. The metric of the other protocol is the sum of all the
    bandwidth to the destination."

    If this sounds familiar to anyone can you please help shed some light.

    Thanks,

    silentbob
     
    silentbob, Aug 8, 2006
    #1
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  2. silentbob

    Merv Guest

    OSPF uses a simple metric based on the sum of the link "cost" to a
    destination. Cost can be manually assigned or automatically set based
    on link bandwidth.


    EIGRP uses a compound metric based on multiple parameters - bandwidth,
    delay, load, reliability and Maximum Transmission Unit (MTU). By
    default, it uses only the bandwidth and delay parameter.
     
    Merv, Aug 8, 2006
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  3. silentbob

    anybody43 Guest

    In both cases to obtain the metric for a destination
    the indvidual link metrics are added together. However see
    the section lower down regarding EIGRP.

    For CCNP you need to be right on top of this. You
    will need to refer to some decent books or the
    Cisco web site.

    I would say that this was guaranteed to come up in the exam.

    I can just see it:-

    Of delay, bandwidth, reliability, ... (I forget), some made up.

    the default metrics used by EIGRP are:-

    A
    B
    C

    OOPS forgetting is a bad idea since I need to do CCNP
    now too.

    http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a00800c2d96.shtml
    EIGRP updates contain five metrics: minimum bandwidth,
    delay, load, reliability, and maximum transmission unit (MTU).
    Of these five metrics, by default, only minimum bandwidth and
    delay are used to compute best path. Unlike most metrics,
    minimum bandwidth is set to the minimum bandwidth of the
    entire path, and it does not reflect how many hops or low
    bandwidth links are in the path. Delay is a cumulative value
    which increases by the delay value of each segment in the
    path. For more information on EIGRP metrics refer to the
    Enhanced Interior Gateway Routing Protocol White Paper.
     
    anybody43, Aug 8, 2006
    #3
  4. silentbob

    silentbob Guest

    Thanks fellas, I remembered now what I was thinking of. EIGRP uses
    the lowest bandwidth link in the path to calculate the metric, wheras
    OSPF uses the sum of the bandwidths. (plus other calculation for both
    protocols.)
     
    silentbob, Aug 8, 2006
    #4
  5. silentbob

    Merv Guest


    Let be very clear about this - OSPF DOES NOT uses the sum of the
    bandwidths - it uses the sum of the link costs - the link costs CAN and
    usually are based on the link bandwidth.


    You can set the OSPF link cost by configuring the "ip ospf cost"
    command on an interface and in so doing give a cost value that is not
    based on bandwidth but someother factor.
     
    Merv, Aug 8, 2006
    #5
  6. silentbob

    J Guest

    I too can guarantee that it will come up because I just took the
    642-801. The intricacies of EIGRP and OSPF easily account for 2/3s of
    the test. This includes the differences, simularities, metrics,
    neighbor relationships, scenarios involving successors or DRs failing,
    etc. I found the IS-IS questions to be fairly simple since I've been
    reading up on it for my own use for a couple months now. They had a
    number of questions and a simulation involving redistribution. BGP
    questions were on the test too, especially pertaining to the
    involvement between the IGP and EGP, how announces what, etc. IPv6 was
    also on the test including an in-depth analysis of the header as well
    as basic functionality. They'll throw a routing table at you and a
    network diagram and ask "what kind of area is area X". You need to
    recognize that area X is a NSSA area based on what's in the routing
    table. They'll give you a network diagram with IP info, tell you that
    it's OSPF, the configs don't specify a loopback and no router-ID is
    configured, that everything was just rebooted, and that the IGP has
    finished it's election processes. They'll then ask you which router is
    the BDR.

    Basically if you aren't very familiar with the finer details of EIGRP
    (they wouldn't shut up about EIGRP), OSPF, IS-IS, iBGP, some eBGP, some
    RIPv2 for comparisons, and IPv6 then you need to study some more. This
    test requires a fair bit of experience and/or lab time.

    Good luck
    J
     
    J, Aug 8, 2006
    #6
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