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Default routes in an area

Answered Question
Dec 11th, 2005
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Hi,


How are the default routes configured in one internal router transmitted to other internal router in an area ? Is it by Type 5 LSA ? Then it should be transmitted to all through the AS as Type 5.Correct me if I am wrong.


Thanks,

Vijay

Correct Answer by pkhatri about 11 years 8 months ago

Hi Vijay,


As soon as an OSPF router originates a type-5 LSA that carries the default route it becomes an ASBR, even if it does not originate any other external routes.


If you try this out on a router, you will see that this router will now set the E-bit in its router-LSA (type-1 LSA), signifying that it considers itself to be an ASBR.


Paresh.

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pkhatri Mon, 12/12/2005 - 20:51
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Hi,


If you inject a default route by means of default-information originate, this will be carried to all non-stub areas in a type-5 LSA.


Paresh.

vpalania Tue, 12/13/2005 - 02:48
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Hi,


For a router to generate Type 5 LSAs, it should be an ASBR.But, how come an ordinary router generate a Type 5 LSA to carry default route ? or Only ASBR router can advertise default route? Correct me if I am wrong.


Thanks,

Vijay

attrgautam Tue, 12/13/2005 - 03:02
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Nopes it is not needed for a Router to be ASBR to generate default. An ABR can generate a NSSA Default ROute ( The route will be N1 and N2 not E1 and E2). A totally stub area would have an IA default Route not an external default route which is genrated with default-information originate

vpalania Wed, 12/14/2005 - 01:32
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Hi,


I am not talking about default route generated by ABR(as Type 3 LSA for Stubby Area and as Type 7 LSA for NSSA).I am just talking about the default route generated by a normal router inside a noram area.

I got the document:

http://cisco.com/en/US/tech/tk365/technologies_configuration_example09186a00801ec9f0.shtml

which says 'Default routes injected into a normal area can be originated by any OSPF router'


Thanks,

Vijay


Correct Answer
pkhatri Wed, 12/14/2005 - 03:03
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  • Purple, 4500 points or more

Hi Vijay,


As soon as an OSPF router originates a type-5 LSA that carries the default route it becomes an ASBR, even if it does not originate any other external routes.


If you try this out on a router, you will see that this router will now set the E-bit in its router-LSA (type-1 LSA), signifying that it considers itself to be an ASBR.


Paresh.

vpalania Wed, 12/14/2005 - 04:11
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Hi,


Thanks for your explanation.


Thanks,

Vijay

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