Default route into NSSA area before completing connectivity to area 0

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Feb 5th, 2008
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We are configuring an OSPF totally NSSA area with two ABR. When one ABR loses connectivity with backbone area (each ABR only has one link to backbone area) , NSSA area internal routers learn only one default route. But I'm noticing when we connect again that ABR to backbone area, it starts to inject default route into NSSA area before reaching OSPF full state in backbone area. Therefore ip flows load balanced to this ABR from inside of NSSA area will fail until this ABR reach OSPF full state.


Is there anyway to workaround this problem, delaying route default injection until ABR gets full state?


Thanks.

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Edison Ortiz Tue, 02/05/2008 - 06:24
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Are you injecting the default-route with default-information originate within the NSSA command -or- via the default-information originate command under the OSPF routing process ?


With the latter command, you can configure a route-map that checks for routes in the RIB before injecting the default-route. That's called conditional advertisement.



HTH,


__


Edison.

jmfranco Tue, 02/05/2008 - 06:42
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Hi Edison.


Thanks for your post.


I'm using a totally NSSA area (area xx nssa no-summary), and default route is injected automatically, without default-information originate command.


Is it possible to configure in this scenary something like you are telling me?


Thanks.

Edison Ortiz Tue, 02/05/2008 - 07:00
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Can you post a portion of the config? I don't think no-summary generates a default route.


Also, post the show ip ospf database.


smitty6504 Tue, 02/05/2008 - 08:41
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I think there is some confusion here. There is not a totally NSSA. There are a totally stuby area and a NSSA (Not so stuby area). I'm going to assume that you are using a totally stuby area. The reason i say that is the ABR injects a default summary link into the area. If there are more then one ABR in the area it will pick the closest ABR as the defaul gateway to inject into the totally stuby area. Please post your config so we can confirm this but that sounds like what is happing.

s.arunkumar Tue, 02/05/2008 - 20:15
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Hi

Now the method u are using injects a type 3 default route into the nssa,and the method edison suggested would inject a type 7 default route into the nssa..


Its surprising that the routes are getting into the routing table before the neighbor state is full..


Try out with injecting type 7 default route as edison suggested

There are commands like "max-metric router-lsa" and "timers spf" etc to delay the ospf route building,but i dont have much experience with this and dont thing will help much in this scenario..

Lets hear what experts have to say on this..


arun

jmfranco Tue, 02/05/2008 - 23:11
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Thanks to everybody for your posts.


I wanted to say totally stub NSSA area, not totally NSSA. By this way, I'm trying to inject deafault routes into the NSSA totally stub area using a type 3 summary LSA's, and all seems to work fine. Take in mind my lab topology in which I'm using R1 and R2 as ABR, and R3 as NSSA area internal router with serial links to R1 and R2. Well R3 learns default route from R1 and R2. Now, let's assume initially R1 link to backbone area is down (ABR's only have one link to ABR). In this situation, R1 is a NSSA area internal router and it doesn't inject default route in NSSA area, only R2 do it.


Now R1 link to backbone goes up. Well, now R1 is an ABR and the problem I'm noticing is when R1 is in initial steps of neighbor adajencies in backbone area (init,2way...), it starts to inject default route in NSSA area before to reach full state for DR and BDR to be able to send traffic properly to backbone.


Here you have my routing protocol configs for R1, R2 and R3:


R1

---

router ospf 1

router-id 10.211.248.252

log-adjacency-changes

area 80 nssa no-summary

network 10.211.248.1 0.0.0.0 area 80

network 10.211.248.252 0.0.0.0 area 80

network 128.90.80.11 0.0.0.0 area 0


R2

---

router ospf 1

router-id 10.211.248.253

log-adjacency-changes

area 80 nssa no-summary

network 10.211.248.9 0.0.0.0 area 80

network 10.211.248.253 0.0.0.0 area 80

network 128.90.80.12 0.0.0.0 area 0


R3

---

router ospf 1

router-id 10.211.248.254

log-adjacency-changes

area 80 nssa

redistribute static subnets

redistribute eigrp 11 subnets route-map EIGRPAOSPF

network 10.211.248.2 0.0.0.0 area 80

network 10.211.248.10 0.0.0.0 area 80

network 10.211.248.254 0.0.0.0 area 80

default-metric 1


router eigrp 11

redistribute ospf 1 metric 2000 10000 255 1 1500 route-map OSPFAEIGRP

network 10.211.248.4 0.0.0.3

no auto-summary


access-list 1 permit 0.0.0.0

access-list 2 deny 0.0.0.0

access-list 2 permit any


route-map OSPFAEIGRP permit 10

match ip address 1


route-map EIGRPAOSPF permit 10

match ip address 2

mohammedmahmoud Wed, 02/06/2008 - 01:19
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Hi,


Lets first agree about the theory behind the default route rules, as it varies depending on multiple factors:


1.In the case of NSSA totally stub area, the NSSA ABR generates a default summary route (type 3 summary LSA).

2.In the case of NSSA, by default the ABR doesn't generate a default summary route it must be manually done following the below 2 rules:

a.If this router is an ASBR, then it won't be able to generate the default route unless it has a default route in its routing table known from another routing protocol (thats why it is injected as Type 7 rather than type 3).

b.If this router is an ABR, then the default route is generated with or without a default route in its routing table.


Now to your case, can you please post some show outputs to clarify your problem, the routers can't exchange LSAs until adjacency is formed.


More over, as for Arun's suggestion for using max-metric (stub router advertisement), i don't believe that this applies to the generated default route, as this feature affects only the transit routes to prevent the router from being a transit router (more precisely being a transit router of last resort by increasing the metric of those routes).


BR,

Mohammed Mahmoud.

jmfranco Wed, 02/06/2008 - 03:06
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One note to Mohamed post:

"b.If this router is an ABR, then the default route is generated with or without a default route in its routing table.". I'm agree with you, but is needed default-information-originate command.

Take a look to attached file with show ip route commands obtainted from R1 and R3 (I suggest you to take a look only to default routes).


thanks.



mohammedmahmoud Wed, 02/06/2008 - 03:24
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Hi,


Exactly, sub-point "b" is part of point "2", and in point 2 i stated that it must be manually configured.


I've something in my mind, let me simulate your case and get back to you.


BR,

Mohammed Mahmoud.



smitty6504 Wed, 02/06/2008 - 05:17
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Sorry. I was looking in my BSCI book and they only talk about NSSA, stub areas, and totally stubby area.

Edison Ortiz Wed, 02/06/2008 - 09:56
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Bryan,


Quite all right, we are all here to learn :)



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