OSPF stub area

Unanswered Question
Feb 25th, 2007

I configured two router as below

RouterA (stub area 1) --> RouterB (stub Area1 & Area0 )

I haven't add default-information originate in RouterB but RouterA can learn a route like this, why it happen like this ?

O*IA 0.0.0.0/0 [110/11] via 172.20.1.9, 00:12:11, Ethernet0

I have this problem too.
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sundar.palaniappan Sun, 02/25/2007 - 07:22

That's the normal behavior. An ABR injects a default route into a stub area. Stub area doesn't accept external routes, aka type 5 LSAs, and they accpet all other LSAs + a default route from the ABR. The default route is advertised as a type 3 LSA, aka IA route. To get to external networks, stub area uses the default route advertised by the ABR.

This link should provide answers on how OSPF generates default routes with different setup.

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094a74.shtml

HTH

Sundar

acbenny Sun, 02/25/2007 - 07:47

Is it mean that default-information originate is used in ASBR only to generate a default route inject into ospf domain, but ABR can automatically generate default route ?

sundar.palaniappan Sun, 02/25/2007 - 08:23

Yes, default route is automatically generated to a stub area. You would need 'default-information originate' only if it's a normal OSPF area and it can be configured on an ABR/ASBR.

HTH

Sundar

"default-information originate" is used to manually advertise a LSA 3 default route if you don't issue "no-summary" command under OSPF process.

In other word, if you issue "no-summary" command under OSPF process, OSPF will advertise a LSA 3 default route automactially.

"no-summary" means filter out LSA 3 and advertise a LSA 3 default route automactially.

PS: "default-information originate" can be used to manually advertise LSA 7 default route on ASBR

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