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Latest post 03-19-2017 6:55 PM by Martinl. 15 replies.
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  • 03-15-2017 1:29 PM

    Feasible Distance/Computed Distance

    Hello,

    in the INE V5 video series dedicated to EIGRP Brian describes Feasible Distance as a current Computed Distance which is wrong based on what I see on the Internet:

    https://supportforums.cisco.com/discussion/12569101/definition-feasible-distance-eigrp-convergence

    So, should we treat Feasible Distance as a Computed Distance or rather a historical record?

    What do we see in routing table? (FD/RD) or (CD/RD)?

    Thanks!

    Filed under:
    • Post Points: 50
  • 03-15-2017 5:38 PM In reply to

    Re: Feasible Distance/Computed Distance

     

    i would go with whatever they (Peter and Narbik)  wrote in ccie rs book (cisco press) for ccie written exam.  same in case of ccna exam, use whatever definition they use in ccna books from cisco press

    • Post Points: 5
  • 03-16-2017 2:36 AM In reply to

    Re: Feasible Distance/Computed Distance

    dmytro_s:

    What do we see in routing table? (FD/RD) or (CD/RD)?

     

    (CD/RD)

     

    But the FD may be equal to  or lower than the CD.

     

    FD is the historical minimum to a destination since the route has entered PASSIVE state.


    Some more info:

     

    https://tools.ietf.org/html/rfc7868

     

    BR,

    Timmy

    • Post Points: 20
  • 03-16-2017 6:56 AM In reply to

    Re: Feasible Distance/Computed Distance

    Narbik operates (CD/RD) terminology whilst Brian believes it's "overcomplicated" and definitely uses FD instead of CD. This is the first time I would disagree with him...

    Thanks guys.

    • Post Points: 5
  • 03-16-2017 11:45 AM In reply to

    • JoeM
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    Re: Feasible Distance/Computed Distance

    dmytro_s:

    So, should we treat Feasible Distance as a Computed Distance or rather a historical record?

    What do we see in routing table? (FD/RD) or (CD/RD)?

    Thanks!

     Hi DMYTRO, Thankyou for this post. I needed to do a little homework to grasp the term "historical record", and this forced me to revise my thinking about the common definition of FD as "the current BEST CD".  

    Peter explains in several threads, but the below statement seems to be the most concise.

    Peter Paluch:
    https://supportforums.cisco.com/discussion/12585006/eigrp-feasible-distance

    ....I wrote the chapter on EIGRP in that book.

    Pages 389 and 390 try to provide the explanation why the Feasible Distance truly is the historical value of the minimum distance to a destination since the last Active->Passive transition....

    Summary Answer:  the FD value does not change until the router begins ACTIVEly querying its neighbors. 

     

     

     

    • Post Points: 20
  • 03-17-2017 12:13 PM In reply to

    Re: Feasible Distance/Computed Distance

    Hi Joe,

    thanks for your message.

    So, we can conclude that FD is a kind of a FIXED best distance to a destination which is calculated after router finishes sending queries and receiving replies in ACTIVE mode. It can be lowered, however, and that's when CD comes into play when the router is in PASSIVE mode. Now CD is the best current distance and FD is the second best distance which will be used in case something happens with a route over CD. Do you agree?

    • Post Points: 20
  • 03-17-2017 1:02 PM In reply to

    • JoeM
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    Re: Feasible Distance/Computed Distance

    Hi Dmytro,

    My thougthts have only changed regarding when the FD reference is updated. This the reason for the "historical FD value".

    I do not see how the FD can be "lowered" without going ACTIVE.

    All of the routes have a Computed Distance, so this term (CD) does not reference a best route.  Variance and Feasible Successor(s) will be based off of the FD (Historical Value). 

    The FD can never be second best.  If the FD route is not being used, then the Feasible Successor will step in as the current best route.  But the FD can never be 2nd best.  It is either alive or not. 

    • Post Points: 20
  • 03-17-2017 2:00 PM In reply to

    Re: Feasible Distance/Computed Distance

    This made my day! 

  • After a topology change and updating RD and CD, find the neighbor providing the least CD
  • Check whether the neighbor providing the least CD is a Feasible Successor
    • If yes, use it right away
    • If not, go Active, even if some other neighbors are Feasible Successors
    • Post Points: 5
  • 03-17-2017 2:19 PM In reply to

    Re: Feasible Distance/Computed Distance

    JoeM:

    I do not see how the FD can be "lowered" without going ACTIVE.

    All of the routes have a Computed Distance, so this term (CD) does not reference a best route.  Variance and Feasible Successor(s) will be based off of the FD (Historical Value). 

    I believe you're right. FD was a best route in the past and can be a reference point when a change on the network happens, but it doesn't mean that feasible successor will have its CD equal to that FD.

    As for a CD term. The EIGRP route that gets installed in routing table has best CD, right? The question is when CD=FD? Is is so unless a change happens and a route goes active? When it's active, it' is getting a new CD which is not equal to the historical FD.  

    • Post Points: 20
  • 03-17-2017 7:09 PM In reply to

    • JoeM
    • Top 10 Contributor
    • Joined on 04-15-2012
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    Re: Feasible Distance/Computed Distance

    dmytro_s:

    I believe you're right. FD was a best route in the past and can be a reference point when a change on the network happens, but it doesn't mean that feasible successor will have its CD equal to that FD.

    I think that you are mixing the terms.  Every possible route for a prefix has a Computed Distance.   But there is only one FD calculated per prefix.  After this, it becomes a Variance issue + Feasible Condition.

    I would expect that a Feasible Successor would not have the same FD as the original Successor. That is why it is a Feasible Successor.

     

    dmytro_s:

    As for a CD term. The EIGRP route that gets installed in routing table has best CD, right? The question is when CD=FD? Is is so unless a change happens and a route goes active? When it's active, it' is getting a new CD which is not equal to the historical FD.  

    In this case, I think that "Historical FD" is not the best term.  Why?  Because a new FD is going to be calculated, and the previous forgotten.

    All CD's (all routes) are considered, since every route (successor or not) has a CD.

     

    • Post Points: 20
  • 03-18-2017 6:07 AM In reply to

    Re: Feasible Distance/Computed Distance

    Well I clrearly realize that every route has its own (CD/RD). But can we define FD as the best CD amongst the available routes' CDs when router is in Passive mode?

    To be precise (as per Peter's explanation), when a router goes active (because it hasn't found the best CD among feasible successors but has found it for a regular route in eigrp topology) and does duffusing computation it installs FD=infinity unless another router "reinitializes" that with a new value. when we have a process like this, in a short period of time we have "hostorical" FD (or the one before router goes Active), then we have FD=infinity and we have a new FD when the diffusing computations are done. That's my current understanding.

     

    • Post Points: 5
  • 03-18-2017 11:12 AM In reply to

    Re: Feasible Distance/Computed Distance

    Reading more posts by Peter I now believe that FD can really be lowered on a router being in a Passive mode:

    "Feasible Distance, or FD, as we have discussed already, is a record of the smallest CD for a particular destination ever encountered since the last time this destination went from Active to Passive state. As opposed to RD and CD that are kept for each neighbor to a destination, FD is kept only for the destination - so regardless how many neighbors you have, a router maintains only a single FD value for a particular destination".

    "Feasible Distance (FD), holding the lowest known value of the CD toward a particular destination since the last time the destination went from Active to Passive state".

    You see that? It's not about computing an FD when being in an Active mode, it's about keeping track of the lowest encountered CD when being in a Passive mode and calling it an FD.

    So, when router is in a Passive mode (not sending Query packets) it still can compute distances towards destinations and once the lowest CD appears (as compared to the current FD) it becomes a new FD.

    I also recall that Brian explained that FD can be lowered, but not increased. Being in the Active mode a new FD can be higher. But being in a Passive mode (Update packest being sent) FD can only be lowered.

    • Post Points: 5
  • 03-18-2017 11:52 AM In reply to

    Re: Feasible Distance/Computed Distance

    And yet another explanation from the book by Narbik/Peter:

    "By its definition, in the Passive state, after the FD has been initialized, it can only decrease (if the current best CD happens to fall below the current value of FD) or remain at its current value (if the current best CD rises but the route remains Passive)".

    So, I was not mistaken, FD can really be lowered even in Passive state.

    • Post Points: 20
  • 03-18-2017 1:46 PM In reply to

    • JoeM
    • Top 10 Contributor
    • Joined on 04-15-2012
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    Re: Feasible Distance/Computed Distance

    dmytro_s:

    And yet another explanation from the book by Narbik/Peter:

    "By its definition, in the Passive state, after the FD has been initialized, it can only decrease (if the current best CD happens to fall below the current value of FD) or remain at its current value (if the current best CD rises but the route remains Passive)".

    So, I was not mistaken, FD can really be lowered even in Passive state.

     

    I think we are chasing our tail on this.     FD is an FD.   One FD per destination.  If something better exists, it would already be the FD.

    How can an FD decrease during passive state, unless...

    .....an additional route has been added?
    .....or unless a current CD has been updated somehow?

    I suppose this could happen, but changing/adding alternative routes isn't the base of our discussion.   It is about what happens when we lose the FD route......when the next best route takes its place. If there had been a better route (CD), it would have become the FD right away, right?

    • Post Points: 20
  • 03-18-2017 2:39 PM In reply to

    Re: Feasible Distance/Computed Distance

    JoeM:

    dmytro_s:

    And yet another explanation from the book by Narbik/Peter:

    "By its definition, in the Passive state, after the FD has been initialized, it can only decrease (if the current best CD happens to fall below the current value of FD) or remain at its current value (if the current best CD rises but the route remains Passive)".

    So, I was not mistaken, FD can really be lowered even in Passive state.

     

     

     

    I think we are chasing our tail on this.     FD is an FD.   One FD per destination.  If something better exists, it would already be the FD.

    How can an FD decrease during passive state, unless...

    .....an additional route has been added?
    .....or unless a current CD has been updated somehow?

    I suppose this could happen, but changing/adding alternative routes isn't the base of our discussion.   It is about what happens when we lose the FD route......when the next best route takes its place. If there had been a better route (CD), it would have become the FD right away, right?

     

    Totally agree on the last statement.

    As for "when FD can change" - it's not only a route (path?) change or a route add. For instance, if someone decreased delay on some of the links forming a current best route (CD), it would trigger an FD decrease despite keeping the same route (path) but changing metrics for better. Probably this is what you mean as well.

    I'm just happier now as I seem to understand the process much better. Thanks for participating in the discussion!

    • Post Points: 20
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