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

    Query about multiple Redistribution Points

    Yan Tian

      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 !

        • 1. Re: Query about multiple Redistribution Points
          Daz_UK

          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 ?

          • 2. Re: Query about multiple Redistribution Points
            Elvin Arias

            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

            • 3. Re: Query about multiple Redistribution Points
              Yan Tian

              Hello Babasdad and Elvin,

              Thank you all for your reply. I am clear about query 1 now. Further clarification on Query 2 is needed though.

               

              From babasdad, it is confirmed that, to reach 10.10.10.0/24 in RIP domain on the left, RD2 will use OSPF E2 route originated from RD1 to replace its own existing RIP route in routing table. Thus, OSPF E2 route will be the only route entry for 10.10.10.0/24

               

              From Elvin, it is indicated that RD2 will save the RD1 redistributed 10.10.10.0/24 route as an alternate route inside it's LSDB. Here, alternate route, does it mean OSPF E2 route will not be the only route entry for subnet 10.10.10.0/24 ?

              Further clarify is grateful.

              • 4. Re: Query about multiple Redistribution Points
                rak

                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

                • 5. Re: Query about multiple Redistribution Points
                  Yan Tian

                  Hello Rak,

                  Thank you for your hints, I think I get it.

                  Alternate route actually means it does exist in OSPF LSDB and will be injected to the routing table immediately if RIP route destined for 10.10.10.0/24 failed.

                  • 6. Re: Query about multiple Redistribution Points
                    rak

                    yup