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Official Lammle User Forum

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  1. #1
    Join Date
    Nov 2011
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    28

    Default Feasible Successor - more or less?

    Well, I'm having some trouble grokking the meaning of feasible successor. Both version 6 and version 7 of the book define a feasible successor in no uncertain terms:

    In order for a route to be a feasible successor, its advertised distance must be less than the feasible distance of the successor route.
    Now, my understanding seems directly at odds with this statement, since the current successor is, in fact, already the route with the least distance. Therefore, a feasible successor must have more distance to the destination network, and not less.

    Would you not agree?

    William

  2. #2
    Join Date
    Dec 2010
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    Left Coast, California
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    Default

    The advertised distance, AD (which is better to call the RD, reported distance, so as not to confuse it with the other AD, administrative distance), meets the feasibilty requirement by having a RD that is less than the current FD - its needs to pass that requirement to be installed as a backup in the topo table.

    I say NEEDS to be smaller because putting a route in the topo table (backup route) that has a RD that is already greater than the currently installed successors total route could lead to a routing loop, if it were installed as the new successor. So the RD must be less than the FD for a route to even be considered as a backup route in the topo table. Again, this is the feasibility requirement.

    I would hit the cisco docs for more details on how routing loops can occur by using a route that has RD > FD (successor route) - because frankly...I can't tell you...I only know they can
    Last edited by ciscodaze; 11-27-2011 at 04:44 PM. Reason: my smiley wont work!!! :-)
    Kevin NET+SEC+A+CCNA
    'All that is not eternal is eternally out of date' ~ C.S. Lewis

  3. #3
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    Nov 2011
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    Default

    Hmm. Seems I may be confusing RD with Administrative Distance.. maybe.

    Still, wikipedia seems to agree with me:

    A feasible successor provides a working route to the same destination, although with a higher distance.
    William

  4. #4
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    Default

    Quote Originally Posted by william View Post
    Hmm. Seems I may be confusing RD with Administrative Distance.. maybe.

    Still, wikipedia seems to agree with me:



    William

    It it wasnt higher, then it wouldnt be in the topology table as a backup...it would BE the successor.
    Kevin NET+SEC+A+CCNA
    'All that is not eternal is eternally out of date' ~ C.S. Lewis

  5. #5
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    Nov 2011
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    Default

    Yes, exactly... yet, the book says it must be less.


    Quote Originally Posted by ciscodaze View Post
    It it wasnt higher, then it wouldnt be in the topology table as a backup...it would BE the successor.

  6. #6
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    Default

    Quote Originally Posted by william View Post
    Yes, exactly... yet, the book says it must be less.
    your are confusing the distance reported by the neighbor (RD) with the total distance from the originating router to the destination router, which includes the jump to the neighbor router PLUS the jump from the neighbor to the destination --- and THAT is going to be larger, if a FS is taken from the topo.

    RD = cost of neighbor to destination network (must be smaller than successor TOTAL distance, and it sits in the topo table as a potential backup)

    FD = the metric of the SUCCESSOR ROUTE, which include the jump from him to the neighbor, PLUS the neighbors jump to the destination network - his RD.

    look at this thread.
    http://www.lammle.com/discussion/showthread.php?t=5538
    Last edited by ciscodaze; 11-27-2011 at 05:14 PM.
    Kevin NET+SEC+A+CCNA
    'All that is not eternal is eternally out of date' ~ C.S. Lewis

  7. #7
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    Nov 2011
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    Default

    Thanks for keeping me on track here... I finally had to go look at the Cisco white paper for EIGRP to get it clear in my head. (Complete with 8x10 color glossy pictures and circles and arrows )

    What I didnt grok is that the comparison is made between a single feasible and a bunch of advertised/reported. Once I got that through my thick head, the book was perfectly clear. And perfectly correct.

    William

    Quote Originally Posted by ciscodaze View Post
    your are confusing the distance reported by the neighbor (RD) with the total distance from the originating router to the destination router, which includes the jump to the neighbor router PLUS the jump from the neighbor to the destination --- and THAT is going to be larger, if a FS is taken from the topo.

    RD = cost of neighbor to destination network (must be smaller than successor TOTAL distance, and it sits in the topo table as a potential backup)

    FD = the metric of the SUCCESSOR ROUTE, which include the jump from him to the neighbor, PLUS the neighbors jump to the destination network - his RD.

    look at this thread.
    http://www.lammle.com/discussion/showthread.php?t=5538

  8. #8
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    Default

    [QUOTE=william;1 9817]Thanks for keeping me on track here... I finally had to go look at the Cisco white paper for EIGRP to get it clear in my head. (Complete with 8x10 color glossy pictures and circles and arrows )


    hey man...if it wasnt for pretty pictures with circles and arrows, Id still be studying for my network+

    a picture is worth a thousand acronyms.
    Kevin NET+SEC+A+CCNA
    'All that is not eternal is eternally out of date' ~ C.S. Lewis

  9. #9
    Join Date
    Nov 2011
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    Oklahoma
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    Default

    Glad I searched the forums for this before posting I have the same question. I am looking over that cisco link now I am following but it does not make total sense though its starting to come together.

    I think I got it now:
    ---------------------------------

    Router chooses the path with the lowest metric. Remember by default EIGRP uses Bandwidth and Delay for its metrics so we look at those.

    There are a bunch of formulas you can get them from the cisco web page they are not to bad to understand but they are something you woould want to look up not try to memorize.

    Basically what happens is you take the Bandwidth + the Delay and you can use yet another formula and figure out the total cost.

    Basically the AD FD is from the neighbor router to the router who is picking which way it wants to go.

    =============== =========
    =============== =============== ==
    =============== =============== =============== =After like 5 edits lol========

    HAHA! I had this all wrong even though I did have pretty darn good logic it was the wrong one I was making it way to complicated this is so easy to understand I am laughing at myself now.

    The trick to all of this is understanding 4 terms and being very careful to understand the english of it all... lol

    First we have, well hex just let me qoute Mr Lammle he could not have said it any better:


    "Feasible distance (FD) This is the best metric among all paths to a remote network, including the metric to the neighbor that is advertising that remote network.

    Reported/advertised distance (AD) This is the metric of a remote network, as reported by a neighbor"


    next we have to look at rules for Successor and Feasible successor I will qoute Mr Lammle again as its pretty much spot on:

    "Successor A successor route (think successful!) is the best route to a remote network."

    "Feasible successor A feasible successor is a path whose advertised distance is less than the feasible distance of the current successor, and it is considered a backup route."

    If you read Mr Lammle qoutes I posted above and think about what it says you dont need the cisco website its not that complex you just have to read those carefully.

    FD is the best route (start to finish) thats what our successor uses and its in the routing table if that goes down if we are using EIGRP we can use the Feasible succesor.

    To be a Feasible successor is simple we are weighing the AD against the FD and the AD better be lower than the FD because if its not we could well have a loop. the AD is the the total path its just what Mr Lammle said in his qoutes the path from the neighbor router reporting it to the remote network.

    The FD is total path from start to finish.
    AD is just from neighbor router to finish.

    Its late and I am not the most efficient writer but I hope I make sense its really simple.




    Respectfully,

    Brian W Catlin
    Last edited by wilder7bc; 07-08-2012 at 12:47 AM.

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