8 Replies Latest reply: Jul 15, 2015 7:08 PM by Kevin Santillan RSS

    Type-4 ASBR summary LSA

    Geo John

      Hello CLN,

       

      Does Type-4 ASBR summary LSA allowed in a stub area? I got confused with one cisco doc What Are OSPF Areas and Virtual Links? - Cisco

       

      Whereas, some article says ABR do not forward Type-4 LSA into stub areas. OSPF area types - PacketLife.net

       

      Regards,

      Geo John

         
        • 1. Re: Type-4 ASBR summary LSA
          Kevin Santillan

          Hello Geo John,

           

          I'd love to answer the question directly but I suggest that you lab this and see the result yourself. When in doubt, always try to lab it up if possible. That way, the information that you'll learn will be retained more effectively. Here is a simple setup that might help you out.

           

          R1-- Area 0 -- R2 -- Area 1 (Stub) -- R3

           

          Create a loopback on R1, give it an address, then redistribute it into OSPF via the redistribute connected subnets command in R1's OSPF process. This makes R1 an ASBR. Next, check if R2 is forwarding a Type-4 LSA into Area 1 and look at R3's LSDB as well to see if it received one. You can then remove the stub configuration for Area 1 or modify its area type and compare the results.

           

          Please share your findings afterwards. If you have questions / concerns when labbing this up, let us know.

          • 2. Re: Type-4 ASBR summary LSA
            Joel Mooke

            Hello Geo

             

            This this is explained in rfc 2328 section 12.4.3. Summary-LSAs

             

            It clearly state type 4 summary lsa should is never advertised into a stub area. Please go through the rfc(not only this particular section) to know ans also to get to know a lot about ospf.

             

            Regards

            • 3. Re: Type-4 ASBR summary LSA
              Sergey

              Juelm,

               

              You've got a typo there the RFC for OSPF is 2328.

              • 4. Re: Type-4 ASBR summary LSA
                Joel Mooke

                Sorry Sergey

                 

                Corrected it

                • 5. Re: Type-4 ASBR summary LSA
                  Geo John

                  Thanks Kevin, R2 is clearly blocking Type-4 ASBR summary LSA and Type-5 External LSA into R3, which is a stub area.

                   

                  I'm sharing my results here:

                   

                  LSDB on R3 ( Standard Area, Loopback configured on R1 )

                   

                  R3#sh ip ospf database

                              OSPF Router with ID (10.0.0.2) (Process ID 100)

                                  Router Link States (Area 1)

                  Link ID         ADV Router      Age         Seq#       Checksum Link count

                  10.0.0.2        10.0.0.2        528         0x80000002 0x00c75d 1

                  192.168.1.2     192.168.1.2     528         0x80000003 0x00c79b 1

                                  Net Link States (Area 1)

                  Link ID         ADV Router      Age         Seq#       Checksum

                  10.0.0.1        192.168.1.2     528         0x80000001 0x00c5a6

                                  Summary Net Link States (Area 1)

                  Link ID         ADV Router      Age         Seq#       Checksum

                  192.168.1.0     192.168.1.2     933         0x80000001 0x00b7cf

                                  Summary ASB Link States (Area 1)

                  Link ID         ADV Router      Age         Seq#       Checksum

                  192.168.1.1     192.168.1.2     187         0x80000003 0x009be7

                                  Type-5 AS External Link States

                  Link ID         ADV Router      Age         Seq#       Checksum Tag

                  5.5.5.0         192.168.1.1     192         0x80000001 0x00fb4a 0

                  R3#

                   

                   

                  LSDB on R3 ( Stub Area, Loopback configured on R1 )

                   

                  R3#sh ip ospf database

                              OSPF Router with ID (10.0.0.2) (Process ID 100)

                                  Router Link States (Area 1)

                  Link ID         ADV Router      Age         Seq#       Checksum Link count

                  10.0.0.2        10.0.0.2        24          0x80000002 0x00c75d 1

                  192.168.1.2     192.168.1.2     24          0x80000002 0x00c99a 1

                                  Net Link States (Area 1)

                  Link ID         ADV Router      Age         Seq#       Checksum

                  10.0.0.1        192.168.1.2     24          0x80000001 0x00864b

                                  Summary Net Link States (Area 1)

                  Link ID         ADV Router      Age         Seq#       Checksum

                  192.168.1.0     192.168.1.2     22          0x80000001 0x00b7cf

                  0.0.0.0         192.168.1.2     22          0x80000002 0x00727e

                  R3#

                  • 6. Re: Type-4 ASBR summary LSA
                    Kevin Santillan

                    Good work. Though I wouldn't say R2 is blocking the Type-4 LSA. Note that Type-4 LSAs are generated by ABRs and not by ASBRs. Hence, R2 is not creating a Type-4 LSA for Area 1 because it won't be of any use since Type-5 LSAs can't be present there. So yes, both Type-4 and Type-5 LSAs are not propagated in Stubs.

                    • 7. Re: Type-4 ASBR summary LSA
                      Geo John

                      You are right Kevin, many thanks for correcting me.

                       

                      So the conclusion: "R2 (ABR) is not creating a Type-4 LSA for Area 1, which is a stub."

                       

                      John

                      • 8. Re: Type-4 ASBR summary LSA
                        Kevin Santillan

                        Correct. You're welcome. In case you want to dig deeper into Type-4 LSAs, have a look at this write up, which is also from Stretch, and try to replicate the scenario.