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OSPF-ISIS redistribution confusion!

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  • 1.  OSPF-ISIS redistribution confusion!

    Posted 08-09-2017 19:24

    Hi,

     

    I was following JNCIE-SP boot camp tasks and I stuck in one task for a while and couldn’t figure out how it goes.

     

    First of all, I draw this simple diagram to elaborate the issue simply as much as I can.

    OSPF-ISIS.PNG

    The task is to aggregate the routes which are advertised by DC coming from OSPF and export it to ISIS level 2.

     

    Note: Wide-metric is configured in level 2 to all external routes will have route preference with 18.

     

    The solution was pretty easy as per steps below:

     

    • Aggregate the routes on both routers R3 and R5 (It has route preference 130)
    • Create policy to export this aggregated routes to ISIS, without matching the protocol, because as they mentioned either R3 or R5 will receive the route from other router (R3/R5) from ISIS with route preference 18 which is better than the locally aggregated routes, so it will be redistributed either from routes coming from ISIS or aggregated one.

    But I got different behavior each time Smiley Sad

     

    • At the beginning I find R3 and R5 only see the aggregated prefix coming from aggregate protocol with route preference 130, so they redistribute and R4 will get two routes and do load balancing between them.
    • Later on, when i deactivate the ISIS and activate it again, I see R5 is receiving the aggregated routes from R3 via ISIS with route preference 18 which is better than the locally aggregated one with 130.
    • As per task both R3 and R5 should export the prefix wherever he receive from, as we didn’t match specific protocol, but I still see R4 in his database that R3 only the one who export this aggregated route, not both.
    • When I deactivate and activate again I got the same scenario as point number 1, and so on.

     

    Sorry for this long discussion!



  • 2.  RE: OSPF-ISIS redistribution confusion!
    Best Answer

     
    Posted 08-10-2017 01:48

    Hi,

     

    "Create policy to export this aggregated routes to ISIS, without matching the protocol, because as they mentioned either R3 or R5 will receive the route from other router (R3/R5) from ISIS with route preference 18 which is better than the locally aggregated routes, so it will be redistributed either from routes coming from ISIS or aggregated one."

     

    When the route is learnt from IS-IS and prefered in the routing-table, the policy is trying to export an IS-IS route from the same level & area back into the same IS-IS database, which I suppose is not accepted as only external routes can be exported to the IGP.  When looking at 'show isis database detail' the aggregate route does not come back into the LSP for that router.

     

    I understand the solution would be configuring the route preference for the aggregate lower than the IGP preference [<18 in this case], thus always exporting the aggregate route into IS-IS DB from R3 & R5 when present.

     

    Cheers,
    Ashvin

     



  • 3.  RE: OSPF-ISIS redistribution confusion!

    Posted 08-10-2017 12:53

    Thanks Ashvin!

     

    This is really what I was thinking about, but what is written and elaborated in boot camp is something different which makes me more confused and stuck on it for long time.