OSPF redistribution into NSSA - diagram is attached.

Answered Question
Jul 8th, 2009

All,

I'm having a hard time understanding something.

If I have a router that is an ASBR and an ABR that lies on area 0 and NSSA area 1. This router is redistributing EIGRP 100 into OSPF. The router in area 1 is seeing the redistributed route as an N2 route.

I have another router in the same topology configured the same way and is redistributing its routes into area 0. I don't see these routes on the area 1 router.

I know that NSSAs only allow type 7s into their areas, and these are converted by the ASBR. I also *think* that type 5s aren't allowed into NSSAs because they're considered a stub area.

So, my question is why is the router that's an ASBR and ABR advertising it's EIGRP route to the NSSA area, but the other router isn't seeing it?

Thanks,

John

I have this problem too.
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Correct Answer by Giuseppe Larosa about 7 years 5 months ago

Hello John,

NSSA areas have been introduced to allow routers in a stub routers to inject = accept external routes.

These routes are injected as O N2 or O N1 by NSSA ASBR and can be converted by NSSA ABR(s) in normal O E1, O E2 routes to be propagated in the OSPF domain in backbone and normal areas.

For this reason an O E2 route generated by top router is not allowed in the NSSA area.

What you see is correct: middle router plays the double role of ABR (NSSA,0) and of ASBR so it is allowed to inject the routes that are locally redistributed as O Nx routes but it can never take from the backbone database an O Ey route to inject it into the NSSA area database.

In other words the translation works only on the opposite direction from NSSA area to backbone area.

Hope to help

Giuseppe

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John Blakley Wed, 07/08/2009 - 08:03

It's actually both. The "top" router is classful, and the "left" router in the middle is classless. Here's the routing table:

"Top" router:

172.15.0.0/24 is subnetted, 1 subnets

O IA 172.15.15.0 [110/20] via 192.168.2.2, 00:04:09, FastEthernet0/0

10.0.0.0/24 is subnetted, 1 subnets

O E2 10.10.10.0 [110/150] via 192.168.2.2, 00:00:21, FastEthernet0/0

C 192.168.1.0/24 is directly connected, Loopback1

C 192.168.2.0/24 is directly connected, FastEthernet0/0

"Middle Router"

172.15.0.0/24 is subnetted, 1 subnets

C 172.15.15.0 is directly connected, FastEthernet0/1

10.0.0.0/24 is subnetted, 1 subnets

C 10.10.10.0 is directly connected, Loopback1

O E2 192.168.1.0/24 [110/150] via 192.168.2.1, 00:03:49, FastEthernet0/0

C 192.168.2.0/24 is directly connected, FastEthernet0/0

"Bottom" Router in nssa:

172.15.0.0/24 is subnetted, 1 subnets

C 172.15.15.0 is directly connected, FastEthernet0/0

10.0.0.0/24 is subnetted, 1 subnets

O N2 10.10.10.0 [110/150] via 172.15.15.1, 00:00:12, FastEthernet0/0

O IA 192.168.2.0/24 [110/20] via 172.15.15.1, 00:02:26, FastEthernet0/0

You can see that the "middle" router has the route from the "top" router. The "bottom" router has the route from the "middle" router (N2 route redistributed), but it doesn't get the 192.168.1.0 route from the "top" router. The only thing that I can think of is because the "middle" router is receiving this from a different source than itself and it's not injecting that route into the nssa because it's learned externally as well. The route that I'm getting on the "bottom" router from the "middle" router is a connected route on the "middle" router.

Thanks,

John

Edison Ortiz Wed, 07/08/2009 - 08:47

Normal behavior - you need to redistribute in the NSSA ASBR - see:

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094a88.shtml

and make note of this paragraph based on the diagram:

Redistributed RIP routes are not allowed in Area 1 because NSSA is an extension to the stub area. The stub area characteristics still exist, which includes no type 5 LSAs allowed.

HTH,

__

Edison.

John Blakley Wed, 07/08/2009 - 10:13

Edison,

you need to redistribute in the NSSA ASBR

I'm not sure I understand what you mean by this. I get the redistribution from the "middle" router, and the only thing I can figure is its because the router is an ASBR and an ABR. On the middle router, if I add "area 1 nssa no-summary" I can see the "top" router with no problem. Is there something else I need to do?

Another question: The "middle" router gets the type 5 lsa from the "top" router, but why doesn't it convert the type 5 into a 7 and send it to the bottom since it is the ASBR for the nssa?

FYI, I'm going to label my routers next time instead of top, middle, left, side, bottom, etc. =)

Thanks,

John

Edison Ortiz Wed, 07/08/2009 - 10:20

You get the external route on the middle router but as illustrated in the link I provided, external routes are blocked - you must redistribute in the NSSA ASBR in order to inject the external routes into the NSSA area.

____

Edison.

Correct Answer
Giuseppe Larosa Wed, 07/08/2009 - 08:53

Hello John,

NSSA areas have been introduced to allow routers in a stub routers to inject = accept external routes.

These routes are injected as O N2 or O N1 by NSSA ASBR and can be converted by NSSA ABR(s) in normal O E1, O E2 routes to be propagated in the OSPF domain in backbone and normal areas.

For this reason an O E2 route generated by top router is not allowed in the NSSA area.

What you see is correct: middle router plays the double role of ABR (NSSA,0) and of ASBR so it is allowed to inject the routes that are locally redistributed as O Nx routes but it can never take from the backbone database an O Ey route to inject it into the NSSA area database.

In other words the translation works only on the opposite direction from NSSA area to backbone area.

Hope to help

Giuseppe

John Blakley Wed, 07/08/2009 - 10:15

Giuseppe,

the translation works only on the opposite direction from NSSA area to backbone area.

I think that's the answer that I'm looking for.

Thanks,

John

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