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613 Views 6 Replies Latest reply: May 21, 2012 10:49 AM by rak RSS

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Query about multiple Redistribution Points

May 19, 2012 3:35 AM

Tian Yan 57 posts since
Apr 24, 2012

I am currently doing some reading on the Multiple Redistribution Points in CCNP ROUTE 642-902 Official Certification Guide.

However, I find myself quite confused on that part. Reading through that chapter leaves me a lot of queries unresolved. It seems that this book only tells you how in a very general way but nothing in details about why. I think I have to refer to CCIE related books for this part. One strange query emerged in my mind is as below,

redistribution.jpg

Query 1: Say 10.10.10.0/24 resides on R7 in RIP domain. After redistributing it to OSPF domain on RD1, RD1 will generate an External OSPF type 2 LSA for 10.10.10.0/24, thus one E2 OSPF route entry destined for 10.10.10.0/24 will be generated on RD1 routing table and replace the existing RIP route entry destined for the same subnet 10.10.10.0/24, as OSPF has a lower AD than RIP. I know this is not gonna happen, but do not know why?

 

Query 2: Redistribution works based on the real route entries in routing table. If several routes from different routing protocols can be used to reach the same subnet, then the one learned from the routing protocols with lowest AD will be selected as the best one and injected into the routing table. So, when RD2, who already has a RIP route entry for 10.10.10.0/24 in its routing table, receives External OSPF type 2 LSA originated from RD1, will RD2 use the External type 2 OSPF route to replace its existing RIP route entry for 10.10.10.0/24 ? What if RD2 also redistributes its own RIP route entry destined for 10.10.10.0/24 into OSPF domain ?

 

Any help is highly appreciated !

  • Daz_UK 136 posts since
    Sep 4, 2009
    Currently Being Moderated
    1. May 19, 2012 4:23 AM (in response to Tian Yan)
    Re: Query about multiple Redistribution Points

    Hi Tian

     

    Query1 :

     

    RD1's routing table will not contain an OSPF route for the 10.10.10.0/24 network because it never receives an ospf route for the  10.10.10.0/24 network . RD1 is redistributing the RIP route into ospf advertising to all the  OSPF routers use me to get to 10.10.10.0/24.

     

    Query 2 :

     

    You are correct the protocol with the lowest AD will win . So when RD2 recieves the OSPF E2 route with A.D of 110 he replaces the RIP route with the superior OSPF route . At this point RD2 no longer has a RIP route for 10.10.10.0/24 in its routing table , so even with redistribution configured the RD2 has no RIP route for 10.10.10.0/24 it now has the OSPF route .

     

    A condition of redistribution is the route must be in the routing table

     

    Hope i helped ?

  • Elvin Arias 1,836 posts since
    Mar 12, 2010
    Currently Being Moderated
    2. May 19, 2012 5:52 AM (in response to Tian Yan)
    Re: Query about multiple Redistribution Points

    When you do redistribution from the RD1 router, unless this router receives the same route from other OSPF router it will not be confused about the 10.10.10.0/24 route the was redistributed by him. On the second part, if RD2 redistribute the same 10.10.10.0/24 route this will break the sub-optimal routing problems that were caused earlier by RD1 redistribution, so in this case RD2 will save the RD1 redistributed 10.10.10.0/24 route as an alternate route inside it's LSDB.

     

    Elvin

  • rak 55 posts since
    Feb 19, 2012
    Currently Being Moderated
    4. May 19, 2012 10:25 AM (in response to Tian Yan)
    Re: Query about multiple Redistribution Points

    hi

    in the second part, if u do something like changing the ad of rip for the 10.10.10.0/24 route  to say 90 then

    there would be two routes from rip with lower ad and form ospf

    in which case the ospf route would just be a backroute

    i guess that wats Elvin meant

  • rak 55 posts since
    Feb 19, 2012
    Currently Being Moderated
    6. May 21, 2012 10:49 AM (in response to Tian Yan)
    Re: Query about multiple Redistribution Points

    yup

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