LSA3 and LSA4 in Lab OSPF Not-So-Stubby Area

I understand that LSA4 provides additional information on how to reach an ASBR in case the ASBR is not located in the same area than the router that is looking for the route to an external destination.

But in the case of this LAB, from the R5's point of view, the information provided by LSA3 and LSA4 should be the same, right? either of them can provides the information to R5 to find the ASBR which is advertising 160.1.6.6, why we need LSA4?

 

For this lab, i do notice that LSA3 does not informs that ASBR can be reached through R3 but LSA4 does inform this option. I wonder why LSA3 does not provide information about R3 but does provide information about R1, since they are seems in the same situation.

 

In R5's LSA3:

R5#sh ip ospf database sum 150.1.6.6

 

            OSPF Router with ID (150.1.5.5) (Process ID 1)

 

                Summary Net Link States (Area 0)

 

  LS age: 1438

  Options: (No TOS-capability, DC, Upward)

  LS Type: Summary Links(Network)

  Link State ID: 150.1.6.6 (summary Network Number)

  Advertising Router: 150.1.1.1

  LS Seq Number: 80000001

  Checksum: 0x678D

  Length: 28

  Network Mask: /32

        MTID: 0         Metric: 11

 

  Routing Bit Set on this LSA in topology Base with MTID 0

  LS age: 1437

  Options: (No TOS-capability, DC, Upward)

  LS Type: Summary Links(Network)

  Link State ID: 150.1.6.6 (summary Network Number)

  Advertising Router: 150.1.4.4

  LS Seq Number: 80000001

  Checksum: 0x40AE

  Length: 28

  Network Mask: /32

        MTID: 0         Metric: 11

 

 

In R5's LSA4:

R5#sh ip os da asbr-sum 150.1.6.6

 

            OSPF Router with ID (150.1.5.5) (Process ID 1)

 

                Summary ASB Link States (Area 0)

 

  Routing Bit Set on this LSA in topology Base with MTID 0

  LS age: 1470

  Options: (No TOS-capability, DC, Upward)

  LS Type: Summary Links(AS Boundary Router)

  Link State ID: 150.1.6.6 (AS Boundary Router address)

  Advertising Router: 150.1.1.1

  LS Seq Number: 80000001

  Checksum: 0x4FA5

  Length: 28

  Network Mask: /0

        MTID: 0         Metric: 10

 

  Routing Bit Set on this LSA in topology Base with MTID 0

  LS age: 1463

  Options: (No TOS-capability, DC, Upward)

  LS Type: Summary Links(AS Boundary Router)

  Link State ID: 150.1.6.6 (AS Boundary Router address)

  Advertising Router: 150.1.3.3

  LS Seq Number: 80000001

  Checksum: 0x994D

  Length: 28

  Network Mask: /0

        MTID: 0         Metric: 20

 

  Routing Bit Set on this LSA in topology Base with MTID 0

  LS age: 1468

  Options: (No TOS-capability, DC, Upward)

  LS Type: Summary Links(AS Boundary Router)

  Link State ID: 150.1.6.6 (AS Boundary Router address)

  Advertising Router: 150.1.4.4

  LS Seq Number: 80000001

  Checksum: 0x28C6

  Length: 28

  Network Mask: /0

        MTID: 0         Metric: 10

Comments

  • JoeMJoeM ✭✭✭

    Hi zyt612,

    Yes.  Two completely different animals (errr LSA's).  ;-)

    For this lab, i do notice that LSA3 does not inform that ASBR can be reached through R3 but LSA4 does inform this option. I wonder why LSA3 does not provide information about R3 but does provide information about R1, since they are seems in the same situation.

    LSA-type-3 is an actual destination route for R6's loopback (not a Router-ID as in the asbr-summary). 

    In your output, the LSA3 is showing output for R6's loopback (150.1.6.6 in area 1).  R3 is not in Area-1.

    But LSA4 is also seen from R3 (area0 and area2), because R6 redistributes into both areas (area1 and area2). 

               lsa3 (R6 loopback0) advertised in area1.

               lsa4 (Router-ID) for both area1 and area2.  Lo100 redistributed into both areas.

  • JoeMJoeM ✭✭✭

    Here are a couple of good articles:

    Why have a type 4 lsa? - networksherpa  with an explanation with almost the same question.  Confusion when the RID and loopback are the same.

    Where are type 4 lsa's generated? -- packetlife  shows that an LSA4 is created in an NSSA even before anything is ever redistributed. I always thought this was interesting.

     

     

     

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