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OSPF ASBR and ABR

Unanswered Question
Mar 5th, 2014
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Hi,

I have a 7206 that was redistributing static and connected subnets into OSPF in area 0.  I have since done some tidying up and have stopped the redistribution.  The router has interfaces in area 0.0.0.0 and also 0.7.0.1, therefore making it an ABR, however, when I do a 'sh ip ospf' the router is saying the following:


It is an area border and autonomous system boundary router


I'm surprized that it still thinks that it is an ASBR since all redistribution has been removed - no static, connected or any other routing protocol being redistributed.


I though that it might be "stuck" in this state since it was a an ABR and ASBR, therefore, I did a "clear ip ospf process" however, that didn't fix it.


Is there a reason for the router still thinking that it is a ASBR or should I look at reloading the router to clear it?


Here's the output from a show ip ospf command:


sh ip ospf 10

Routing Process "ospf 10" with ID x.x.x.x

Start time: 00:00:16.216, Time elapsed: 1y31w

Supports only single TOS(TOS0) routes

Supports opaque LSA

Supports Link-local Signaling (LLS)

Supports area transit capability

It is an area border and autonomous system boundary router

Redistributing External Routes from,

Router is not originating router-LSAs with maximum metric

Initial SPF schedule delay 5000 msecs

Minimum hold time between two consecutive SPFs 10000 msecs

Maximum wait time between two consecutive SPFs 10000 msecs

Incremental-SPF disabled

Minimum LSA interval 5 secs

Minimum LSA arrival 1000 msecs

LSA group pacing timer 240 secs

Interface flood pacing timer 33 msecs

Retransmission pacing timer 66 msecs

Number of external LSA 1021. Checksum Sum 0x55AA38D

Number of opaque AS LSA 0. Checksum Sum 0x000000

Number of DCbitless external and opaque AS LSA 0

Number of DoNotAge external and opaque AS LSA 0

Number of areas in this router is 2. 2 normal 0 stub 0 nssa

Number of areas transit capable is 0

External flood list length 0

IETF NSF helper support enabled

Cisco NSF helper support enabled

    Area BACKBONE(0.0.0.0)

        Number of interfaces in this area is 4 (1 loopback)

        Area has no authentication

        SPF algorithm last executed 01:25:17.896 ago

        SPF algorithm executed 4 times

        Area ranges are

        Number of LSA 80. Checksum Sum 0x403239

        Number of opaque link LSA 0. Checksum Sum 0x000000

        Number of DCbitless LSA 0

        Number of indication LSA 0

        Number of DoNotAge LSA 0

        Flood list length 0

    Area 0.7.0.1

        Number of interfaces in this area is 6

        Area has no authentication

        SPF algorithm last executed 01:25:17.904 ago

        SPF algorithm executed 4 times

        Area ranges are

        Number of LSA 44. Checksum Sum 0x210C38

        Number of opaque link LSA 0. Checksum Sum 0x000000

        Number of DCbitless LSA 0

        Number of indication LSA 0

        Number of DoNotAge LSA 0

        Flood list length 0



Any advise would be greatly appreciated.

Jonathan.

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Peter Paluch Wed, 03/05/2014 - 14:50
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Hi Jonathan,


Is this router perhaps configured with default-information originate? If yes then that is the reason why your router treats itself as an ASBR. Injecting a default route into OSPF thanks to this command is done by originating an LSA-5 with the 0.0.0.0/0 network, and by definition, any router originating LSA-5 is an ASBR.


Best regards,

Peter

jonathan johnson Wed, 03/05/2014 - 15:02
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Hi Peter,

Yes, it is configured with default-information originate.  I wasn't aware that having this configured would make the router an ASBR, but now knowing how it injects the default route - it makes sense. 

Perhaps I need to use BGP to send the default route to my other routers. 

I need this router to be an ABR so that I can do summarization.

Thanks for assistance Peter.

Jonathan.

Peter Paluch Wed, 03/05/2014 - 15:09
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Hi Jonathan,


You are heartily welcome.


I wonder - why do you consider using BGP to advertise the default route? Is there any problem with this router being a formal ASBR?


Best regards,

Peter

jonathan johnson Wed, 03/05/2014 - 15:43
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Hi Peter,

I was hoping to summarize routes on this router which is why I wanted it not to be an ASBR.  Thinking about it more I don't think I will use BGP to inject a default route.  I might need have a think about the overall design.

Regards,

Jonathan

Richard Burts Wed, 03/05/2014 - 19:46
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Jonathan


In reading this thread it seems that you believe that a router can not/will not do summarization if it is an ASBR. Is that what you believe? If so then on what basis do you believe this?  It is my belief that as long as a router has at least one interface in area 0 and at least one interface in some other area (this is the basic definition of an ABR) that it can generate summaries of routes in one area to other areas. I am not aware of any restriction based on being an ASBR.


HTH


Rick

Peter Paluch Wed, 03/05/2014 - 21:56
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Hi Rick,

You are completely right, there are absolutely no restrictions (not even interactions that I would be aware of) for an ASBR to also act as an ABR and perform inter-area prefix summarization and filtering.

Best right,
Peter

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jonathan johnson Sat, 03/08/2014 - 14:21
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Thanks for your posts Peter and Richard.  It looks like I need to spend some time to revisit my understanding of OSPF areas and associated router functions.

Thanks again,

Jonathan

Richard Burts Mon, 03/24/2014 - 05:21
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Jonathan

 

Have you come to any resolution about this question?

 

HTH

 

Rick

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