OSPF Redistribution Question

[Nigel Pyne]

Router A is an ASBR in a NSSA. Redistributing routes from RIP into OSPF which includes a default route.

Router B is the ABR between the NSSA and Area 0. It receives all the redistributed routes as LSA Type 7.

Router C is another ABR between Area 0 and Area 1. It receives all the redistributed routes from Router A as LSA Type 5. . . bar the default route. The Link State Database has no record of an LSA for the default route.

Is this normal operation? I need a categorical answer on this please.

If it is normal operation, is the 'default-information originate' command required on Router B?

Thanks

[Peter Paluch]

Hi Nigel,

A default route cannot be redistributed into Cisco's implementation of the OSPF protocol. In other words, the default route from RIP will not be redistributed to the OSPF database on Router A while other RIP routes will. The only way to inject a default route into OSPF is to use the default-information originate command in regular areas, or using the command area X nssa default-information-originate in NSSA areas, where X is the area number.

Therefore - yes, this is a normal behavior for Cisco OSPF.

In your case, the area X nssa default-information-originate command should be used on the Router A.

Best regards,

Peter

[Nigel Pyne]

Hi Peter

Thanks for the reply.

What I'm observing doesn't fully tally with your explanation. It might just be that you confused the routers in your explanation - Router A and B.

I do see the default route in the ospf routing table in Router A and Router B. Router B has the default route as an LSA Type 7 in its LSA Database and the default route is in its routing table. It's just Router C that doesn't see it!

To clarify, Router A is connected to the RIP domain and the NSSA Area - let's call it Area 2.

Router B is the ABR between Area 2 and Area 0

Router C is an ABR between Area 0 and another area - we'll call it Area 1.

So it appears that the default route is being redistributed but being stopped at Router B - the ABR between the NSSA and Area 0. So the area x nssa default-information-originate command should be configured on Router B and not Router A?

[Peter Paluch]

Nigel,

This is strange because I have verified my config on 2691 IOS and a default route learned via RIP is not being redistributed into OSPF, not even in a NSSA area.

Can you post the OSPF configuration from the Router A, plus its routing table and its LSA database output?

Best regards,

Peter

[Nigel Pyne]

I will get this for you. What you're not going to like is that up until a week ago Router C was seeing the default route as a Type 5 in its LSA Database and thereby it got populated into its routing table! No config changes made and now Router C isn't seeing it.

I'll get the configs asap.

[Peter Paluch]

Nigel,

Thanks - unfortunately, for upcoming 3 hours, I will be off. I'll get back after that - in the meantime, perhaps some of other friends here will take over.

Until then!

Best regards,

Peter

P.S.: Yes, you are right - I don't like it

[Jon Marshall]

Nigel

So the area x nssa default-information-originate command should be configured on Router B and not Router A?

No it should be configured on router A ie. the one doing the redistribution between RIP and OSPF.

I have just labbed this up to confirm and as Peter said without the area x nssa default-information-originate command you should not see a default-route on either B or C.

Jon

[Nigel Pyne]

Thanks. I'm still waiting on the configs from a colleague. However, we may be able to settle this without the configs. It may be that the area x nssa default-information-originate command is there on Router A. Won't know until I get the configs.

The real question is how do I get Router C to see the default route and get it into its routing table?

Nigel

[Jon Marshall]

Nigel

Once the "area x nssa default-information originate" command is applied on A then the default-route should be in router C's routing table.

Jon

[Nigel Pyne]

But it isn't. We see it in Router B but not Router C.

So if the 'area x nssa default-information-originate' command is on Router A then the fact that Router C doesn't see the default route I can conclude that I have a genuine issue?

[Jon Marshall]

Nigel

Yes, if you cannot see it on Router C you have an issue.

We need to see the configs really to be sure.

Jon

[Peter Paluch]

Jon and Nigel,

Jon, thanks for taking this issue up while I was unavailable. I hope you'll stay active in this thread - the issue seems to be quite intriguing.

Nigel, the configuration on router A plus the output of its link-state database will be crucial.

Best regards,

Peter

[manish arora]

Can you also get the "show ip ospf database nssa-ext" output from the Router B ( ABR for NSSA ) . As far as the nssa default-information-originate command is concerned , it should be on the Router A as Jon & Peter already mentioned.

If the command  " area XX nssa default-information-originate" is configured on the Router B in your case then you will see the default route being propagated in the NSSA as type 7 even if the Router B does not have that route in its routing table. but the output of the above command will show you that "NO TYPE 7/5 TRANSLATIONS" .

Manish

[Peter Paluch]

Manish,

but the output of the above command will show you that "NO TYPE 7/5 TRANSLATIONS" .

Hmmm, correct me if I am wrong but this line should not be related to the area X nssa default-information-originate command at all. A NSSA area has only one router performing the 7-to-5 translation, even if it has multiple ABRs. As per RFC 3101, the translator is the ABR with the highest RID. Other ABRs will not be 7-to-5 translators and will produce th e output similar to yours.

That is how I have always understood this indication.

Best regards,

Peter

[manish arora]

Here an example :-

1> Cisco's example where the ABR does not have the default route but is still origanating the default info in the NSSA , the output shows "NO type 7/5 translation"

http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a00801ec9fa.shtml#exam

2> some other example :-

where the originate command is on the ASBR , the ABR shows 7/5 translation in options field :-

http://wiki.nil.com/External_default_route_in_NSSA_area

I hope I understood it well ;-)

Manish