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Default route in OSPF stub and totally stub area when we have 2 ABR

Ravi Pande
Beginner
Beginner

Hello,

I was reading OSPF stub and totally stub area and see that there is default route pointing to ABR when we configure an OSPF area as stub or totally stub.
However, in my topology I tried 2 ABR's and see that there is only one default route pointing to only one ABR. So is it like something there is an election for what ABR will be used in the default route or there should be 2 default routes pointing to both the ABR's?

Best Regards,
Ravi

7 Replies 7

Reza Sharifi
Hall of Fame Master Hall of Fame Master
Hall of Fame Master

Hi,

You will see one in the routing table as the preferred route but if you look at the OSPF database you should see 2 default routes (sho ip ospf data summary).

HTH

Thank you for your reply @Reza Sharifi. If I have 2 equal cost default route to ABR then  in routing it selects the default route to ABR based on higher router ID?

Hi,

Couldn't see the diagram you posted but assuming each connection to the ABR is a point-to-point connection, that is correct.

HTH

Apologies for replying late,I was out of town. Please,find below the screenshot.And its not point to point link. Its a broadcast type network.
screeenshot.PNG

I can now see your diagram (or at least most of it - the addressing is clipped off for the networks going out the right side but I suspect that part is not important for this discussion). But there is not anything in the diagram that tells me how OSPF is set up. Where are the OSPF areas? Which routers are ABR? What router are you looking at to see default routes?

 

I agree with Reza that if an area is stub or totally stub and has 2 ABR that each ABR should be advertising a default route. Can you post the output of    sho ip ospf data summary.

 

HTH

 

Rick

HTH

Rick

Hello Rick thanks for your reply. Please, find attached the complete topology and command output in txt file.

Thanks,
Ravi

 

Thank you for the output. It does clearly show that the OSPF data base does have two entries for the default route. One advertised by 3.3.3.3 and the other advertised by 4.4.4.4

  LS age: 542
  Options: (No TOS-capability, DC, Upward)
  LS Type: Summary Links(Network)
  Link State ID: 0.0.0.0 (summary Network Number)
  Advertising Router: 3.3.3.3
  LS Seq Number: 80000001
  Checksum: 0x57DA
  Length: 28
  Network Mask: /0
        TOS: 0  Metric: 1

  Routing Bit Set on this LSA
  LS age: 187
  Options: (No TOS-capability, DC, Upward)
  LS Type: Summary Links(Network)
  Link State ID: 0.0.0.0 (summary Network Number)
  Advertising Router: 4.4.4.4
  LS Seq Number: 80000003
  Checksum: 0x35F6
  Length: 28
  Network Mask: /0
        TOS: 0  Metric: 1

 

There is only a single entry in the IP routing table and that is expected. In this case there are two entries for the default route but they are not equal cost. One is learned from an ABR one hop away and the other is learned from an ABR that is two hops away.

 

HTH

 

Rick

HTH

Rick
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