7 Replies Latest reply: May 11, 2012 11:00 AM by C1SC0M - CCNP,CWNA,Net+ RSS

    Eigrp, OSPF summarization dilemma

    PRASH

      I'm stuck on two issues and they involve summarization at the ASBR. I've attached a picture of the topology. If anyone can help me out and explain wha I did wrong and what I should have done that would be brilliant.Thanks

       

      I summarized the 4 networks belonging R4 into 4.0.0.0/18 AND

      then issued the summary-address command on R3 into the OSPF

      process.R1 and R2 BOTH learned the 4.0.0.0/18 as external

      routes. However for some strange reason R4 and R5 have also learnt

      the 4.0.0.0/18 route and have learnt it externally via EIGRP.

       

      Conversely i have a similar problem when i summarized the loopbacks on R2

      into 2.0.0.0/14 AND used the ip summary-address command on interfaces

      fa1/0 and fa0/1. r4 and r5 learn the summary which is great! but when i checked

      R2 i saw this:

      C       2.2.0.0/16 is directly connected, Loopback2

      C       2.0.0.0/16 is directly connected, Loopback0

      O E1    2.0.0.0/14 [110/201] via 10.23.0.3, 00:06:05, FastEthernet0/1

      C       2.1.0.0/16 is directly connected, Loopback1

           4.0.0.0/18 is subnetted, 1 subnets

       

      Why is R2 receiving 2.0.0.0/14 as an external route? I issued the ip summary-address command into the connected interfaces belonging to R3 which go into EIGRP.

        • 1. Re: Eigrp, OSPF summarization dilemma
          Brian

          put the 4.0.0.0/18 summary on R4 f0/0 towards R3.  R3 should then have the EIGRP routes

           

          4.0.0.0/18 from R4

          5.5.5.0/24 from R5

           

          you will advertise this into OSPF on R3.  Don't forget the subnets keyword as this is not a classful summary.

           

          since the OSPF area is only area 0, you don't have an ABR.  you have an ASBR in R3.

           

          redistribute the OSPF routes into EIGRP on R3.  Then on R3, put the 2.0.0.0/14 summarry only on f0/1 towards R4 and f1/0 towards R5.

           

          you will need to use a route map for both the redistribute commands to filter the original routes back into their original domain.

           

          Brian

           

          • 2. Re: Eigrp, OSPF summarization dilemma
            PRASH

            HI Brian, thanks a lot for your help. I think I got it working now. Well it did resolve it but i hope the syntax was OK. I summarized the 4.0.0.0/18 on R4 as you suggested.

             

            I then used a prefix list and distribute list on R3 just to see if that would work and it did.

             

            R3(config)#ip prefix-list NETWORK-2 deny 2.0.0.0/14

            R3(config)#ip prefix-list NETWORK-2 permit 0.0.0.0/0 le 32

            THIS got rid of the external route to 2.0.0.0/14 in R1 and R2's routing table and also there won't be any external type

            5 LSAs for this network

             

            Alternatively I also used a route map and linked it to a Access-list. Issued on R3

                10 deny   2.0.0.0, wildcard bits 0.3.255.255 (1 match)

                20 permit any (4 matches)

            Then create a route map on R3:

            route-map EIGRP>OSPF, permit, sequence 10

              Match clauses:

                ip address (access-lists): 1

            ON R3 I issued  redistribute eigrp 1 metric 200 metric-type 1 subnets route-map EIGRP>OSPF

             

            This again also seemed to be a viable solution. But would another way of been using tags? Tagging the routes I don't want to be redistributed back into the routing domain they came from? is this better?

            • 3. Re: Eigrp, OSPF summarization dilemma
              sparky

              Hi Prash,

               

              In my blog i have done the CBT nugget labs and you will find tagging with route-maps on this page (between OSPF and EIGRP):

              http://bring-on-the-ccnp.blogspot.co.uk/2012/03/ipv4-redistribution-implementing-simple.html

              • 4. Re: Eigrp, OSPF summarization dilemma
                C1SC0M - CCNP,CWNA,Net+

                Hi,

                Using Tags definitely is a more elegant solution:

                 

                 

                 

                Use these route maps in your redistrubute statements.  With tagging you won't need to creat ACL in order to filter the prefixes.

                 

                 

                 

                route-map EIGRP-into-OSPF permit 10

                set tag 20

                 

                route-map EIGRP-into-OSPF deny 20

                match tag 40

                -------------------------------------------------------------

                route-map OSPF-into-EIGRP permit 10

                set tag 40

                 

                route-map OSPF-into-EIGRP deny 20

                match tag 20

                 

                BTW the thread should said Redistribution Dilemma, your summarization is OK

                • 5. Re: Eigrp, OSPF summarization dilemma
                  jean-christophe manciot - CCDP

                  Your solution is elegant, indeed.

                  Just a slight typo: it would work better if you switched the statements in the route maps

                  • 6. Re: Eigrp, OSPF summarization dilemma
                    PRASH

                    Erm do you mean deny first then permit?

                    • 7. Re: Eigrp, OSPF summarization dilemma
                      C1SC0M - CCNP,CWNA,Net+

                      HI guys,  just to clarify.  Yes, the DENY statements should be first.  I mis-typed that.  The other thing, for some reason I was thinking of double redistribution when I suggested this, and this would prevent loops.  But here that is not the case.

                       

                      In these scenario even do you don't filter anything you won't have loops.  Now I'm looking into it.  Hope to come back soon with a correct explanation.