OSPF E2 type route

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Feb 10th, 2008
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I am a bit confused with the ospf type E2 route. As per the doc when a route is marked as type E2, it does not calculate the metric to the ASBR



Then referring to the attached diagram, why isn't that iam seeing 2 routes for the subnet 1.1.1.0/24 and the drfault 0.0.0.0


R2 and R4 have been configured with default-information originate command

At R3 I see only one default route from R2.


Ambi



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Correct Answer by Edison Ortiz about 9 years 1 month ago

Ambi,


What you are seeing is called recursive-routing. In order for R3 to know about routes from R4, the cost is 2 from R3 (see show ip route)

O 20.20.20.4 [110/2] via 30.30.30.2


while in order for R3 to know about routes from R2, the cost is 1 - the cost of the fastethernet interface.


Increasing the cost on the fastethernet interface on R3 that is facing R2 (ip ospf cost 2) should provide the desired result.


HTH,


__


Edison.

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Edison Ortiz Sun, 02/10/2008 - 07:22
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Can you post the show ip ospf database from R3 ?


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Edison.


ambi@cisco Sun, 02/10/2008 - 08:44
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Edison,


The database shows that i learn two external routes..its only that the RT does not show it


I have attached the configs and the show ip ospf database of all the routers


Ambi



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Edison Ortiz Sun, 02/10/2008 - 09:16
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Ambi,


What you are seeing is called recursive-routing. In order for R3 to know about routes from R4, the cost is 2 from R3 (see show ip route)

O 20.20.20.4 [110/2] via 30.30.30.2


while in order for R3 to know about routes from R2, the cost is 1 - the cost of the fastethernet interface.


Increasing the cost on the fastethernet interface on R3 that is facing R2 (ip ospf cost 2) should provide the desired result.


HTH,


__


Edison.

Edison Ortiz Sun, 02/10/2008 - 09:54
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> Increasing the cost on the fastethernet interface on R3 that is facing R2 (ip ospf cost 2) should provide the desired

> result.


Looking back, I'm not 100% sure about this statement as R3 sees R2 via connected so OSPF does not play a part of the cost here. Please test and let me know.


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Edison.

royalblues Mon, 02/11/2008 - 00:04
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Edison is absolutely correct that the behavior is due to recursive routing.


I managed to lab the setup and it works


you should see the two routes in the rotuing table as required if you increase the ospf cost to 2 between R2 and R3


HTH

Narayan

ambi@cisco Mon, 02/11/2008 - 00:21
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Thanks Edison and narayan


But one thing i am not able to understand is the use of E2 routes cos once you redistribute a route as E2 just to find out later that it recursively uses the path cost again.


It would make more sense to use it as E1 from the beginning


Ambi

s.arunkumar Mon, 02/11/2008 - 00:43
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Hi

Nice topic.. :)


To add to edison point

"If the external routes are both type 2 routes and the external costs to the destination network are equal, then the path with the lowest cost to the ASBR is selected as the best path.""


REF:: http://www.cisco.com/warp/public/104/1.html#t32


arun

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