what is the solution for EIGRP Black Hole !?



in this topology:













Both A and B advertised summary of to X. 

X has better route to through A. if link A to C got down, this advertisement will not reach X. so X will continue send traffic to A, and the traffic get blocked there.

what is the solution to this scenario to let X re-route to B 


  • Explain yourself better - what's the discard route in the diagram? Explain why a simple reconvergence wouldn't fix your ill.

    I can read between the lines and think that I know what you're after (the answer is no, a routing protocol is not going to be smarter than an ACL), but you're better off cleaning up your question.

  • I think this issue would happen with any routing protocol right?  Not just EIGRP.

    I think the discard route being referred to is the standard route to NULL0 that gets automatically created by the router originating the summary as a loop prevention mechanism.

    The root of the problem, as I see it, is that A has a single point of failure when reaching the C, D, and E subnets.  So, of course, if A is advertising a summary and A looses one of those links then it's going to blackhole the traffic for sure. 

    The only way that I can think of to fix this is to install a redundant additional link between A & B so A has an alternate path to C, D & E and eliminate the single point of failure.  Otherwise, you would have to come up with some convoluted tracking object that if any of the links between A and C, D, & E go down then stop advertising the summary.


    I'm curious if anyone has any other more "elegant" solutions.



  • JoeMJoeM ✭✭✭

    As long as other specific routes still exist for that summary, then the Discard Route will remain and continue to be advertised.    Oudmaster is trying to find a way around this issue.....and he actually wants RouterX to recognize the specific network change and then react by going directly through RouterB.

    Possible Solutions:

    • Don't summarize where there is an alternate router needed as a backup for a specific route.
    • Cable between RouterA and RouterB. Let them back each other up indirectly.
    • Use the LEAK-MAP option, to exempt any specific networks from the summary route.
    • ...EEM or some sort of SDN

      • to get the more specific route on RouterB
      • or change metrics for the summary-routes going out.

    The easiest solution is probably to place a connection between RouterA and RouterB.  The blackhole issue will be gone, because RouterA will still recieve the more specific backup route from RouterB.  RouterX will still use the same route, oblivious to how it is being routed.  Magic.

    EEM and/or LEAK-MAP would be the most fun....but would need to be applied on both RouterA and B to gain the effect.




  • Maybe I am missing something here, but if the discard route was generated due to eigrp summary commands and the link between A and C goes down, wouldn't A learn about via D or E? 


    So traffic would go X->A> (D or E) >B>C?  So the traffic would not be black holed unless D & E are stub? 


    If D and E have to be stub, the best solution I agree Joe, put a back to back in A<->B and do not summarize on that interface.  If that is not an option then you would need a leak map. 


    If those are not an option you could do a GRE tunnel for an indirect link between A<->B or use EEM like Joe said.  Or keep it simple and do not summarize.

  • JoeMJoeM ✭✭✭

    Yes, you are correct. It would not be blackholed unless the spokes were stubs.

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