What are LSA Type 4 for?

Hi Guys.

I am wondering what LSA type 4 are for?

I just dont get it althought there are a lot of explanations.

Why do we need an LSA that is generated by an "ABR" and advertises an "ASBR-summary" routes? We have Type 5 for advertising external routes and we have Type 3 for advertising networks between areas.

 

Maybe anyone has a good simple explanation that even I understand :D.

 

Thanks in advance!

 

-m-

Comments








  • If redistribution is happening in a different area than you are in, you know the routes that the ASBR is redistributing, you know who the ASBR is, but you don't know which ABR you should use to reach
    that ASBR.  The reason is that the SPF calculation is only used for devices in your own area.  Inter-area routing is distance vector like.

     

    The details of how it works are covered extensively in the CCNP R&S Class-on-Demand, the CCIE R&S Advanced Technologies Class-on-Demand, and the CCIE R&S Lab Workbook Volume 1.

     

     

    HTH,

     


    Brian McGahan, CCIE #8593 (R&S/SP/Security)

    [email protected]

     

    Internetwork Expert, Inc.

    http://www.INE.com

    Online Community: http://www.IEOC.com

    CCIE Blog: http://blog.INE.com






    From: [email protected] [[email protected]] On Behalf Of zool85 [[email protected]]

    Sent: Saturday, November 05, 2011 4:28 PM

    To: Brian McGahan

    Subject: [CCIE R&S] What are LSA Type 4 for?





    Hi Guys.

    I am wondering what LSA type 4 are for?

    I just dont get it althought there are a lot of explanations.

    Why do we need an LSA that is generated by an "ABR" and advertises an "ASBR-summary" routes? We have Type 5 for advertising external routes and we have Type 3 for advertising networks between areas.

     

    Maybe anyone has a good simple explanation that even I understand :D.

     

    Thanks in advance!

     

    -m-








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  • Hi zool,

      This is also very good explained in Volume 1 workbook; also following blog post explains OSPF external route path calculation:

    http://blog.ine.com/2011/04/04/understanding-ospf-external-route-path-selection/

    On a short description: you are a router in a non-backbone area and the ABR gives you information about an non-OSPF prefix (type 5 LSA, E1 or E2); the ABR gives you the prefix and the advertising router of the prefix in the type-5 LSA, so you need a way to reach the advertising router which is atually the ASBR; because you have no knowledge of that ASBR (not in the same area as you are), OSPF inter-area algorithm for path calculation is distance-vector like, the ABR gives a type-4 LSA to let you know how you can reach the ASBR, that is through the ABR.

    Good luck with your studies!

  • Hi Zool85,

    I hope this example helps to clear your doubt about LSA 4:

    I have R1-RIP-R2(Area 0)-R3-(Area 1)R4 topology:

    R2 is performing redistribution:

    R2(config-router)#do show run | section router ospf
    router ospf 1
     log-adjacency-changes
     redistribute rip metric 1 subnets
     network 2.2.2.2 0.0.0.0 area 0
     network 23.23.23.2 0.0.0.0 area 0

    R4#show ip ospf database

                OSPF Router with ID (4.4.4.4) (Process ID 1)

                    Router Link States (Area 1)

    Link ID         ADV Router      Age         Seq#       Checksum Link count
    3.3.3.3         3.3.3.3         1289        0x80000002 0x005AEA 1
    4.4.4.4         4.4.4.4         1288        0x80000003 0x001BFF 2

                    Net Link States (Area 1)

    Link ID         ADV Router      Age         Seq#       Checksum
    34.34.34.4      4.4.4.4         1288        0x80000001 0x005152

                    Summary Net Link States (Area 1)

    Link ID         ADV Router      Age         Seq#       Checksum
    2.2.2.2         3.3.3.3         1305        0x80000001 0x00E640
    3.3.3.3         3.3.3.3         1345        0x80000001 0x00AE75
    23.23.23.0      3.3.3.3         1345        0x80000001 0x00F9F0

                    Summary ASB Link States (Area 1)

    Link ID         ADV Router      Age         Seq#       Checksum
    2.2.2.2         3.3.3.3         1295        0x80000001 0x00CE58

                    Type-5 AS External Link States

    Link ID         ADV Router      Age         Seq#       Checksum Tag
    1.1.1.1         2.2.2.2         1303        0x80000001 0x00BEE8 0
    12.12.12.0      2.2.2.2         1303        0x80000001 0x003B4C 0

    Summary ASB Link States (Area 1) - This is a  LSA 4 where
    2.2.2.2 is the router acutally performing redistribution and 3.3.3.3 is the router
    origiating these redistributed external routes into this area. If you look on LSA-5 output ADV router of 1.1.1.1 and12.12.12.0 network is 2.2.2.2 and to get rotuer 2.2.2.2, you should go via router 3.3.3.3(Which information is on LSA 4).

    One short answer, how to get 2.2.2.2 router from this Area 1 is via router 3.3.3.3.

    R4#show ip ospf database external 1.1.1.1

                OSPF Router with ID (4.4.4.4) (Process ID 1)

                    Type-5 AS External Link States

      Routing Bit Set on this LSA
      LS age: 1747
      Options: (No TOS-capability, DC)
      LS Type: AS External Link
      Link State ID: 1.1.1.1 (External Network Number )
      Advertising Router: 2.2.2.2 : Who is the actual route originator (performing redistribution)
      LS Seq Number: 80000001
      Checksum: 0xBEE8
      Length: 36
      Network Mask: /32
            Metric Type: 2 (Larger than any link state path)
            TOS: 0
            Metric: 1
            Forward Address: 0.0.0.0
            External Route Tag: 0
    R4#show ip ospf database router 3.3.3.3

                OSPF Router with ID (4.4.4.4) (Process ID 1)

                    Router Link States (Area 1)

      Routing Bit Set on this LSA
      LS age: 1274
      Options: (No TOS-capability, DC)
      LS Type: Router Links
      Link State ID: 3.3.3.3
      Advertising Router: 3.3.3.3
      LS Seq Number: 80000002
      Checksum: 0x5AEA
      Length: 36
      Area Border Router
      Number of Links: 1

        Link connected to: a Transit Network
         (Link ID) Designated Router address: 34.34.34.4
         (Link Data) Router Interface address: 34.34.34.3
          Number of TOS metrics: 0
           TOS 0 Metrics: 1

    HAPPY STUDY

    [:D]

  • Hey! Thanks for the good answers. Think I got it now!

    Will test it in my lab tonight or at the weekend!

    Thanks a lot!

     

  • Watch the Schindler's List.

     

    http://www.imdb.com/title/tt0108052/

     

    Schindler is ABR.

     

    What he advertised to inmates was the information of the outside.

     

    This is LSA4.

    [:D]

     

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