how to filter out default route in OSPF totally stub area?

[zhichao]

Hi

We have a totally stubby NSSA configured. Is there a way we can block the default route generated by the ABR? The reason is we need the routers to learn the default from another ASBR instead of this ABR.

Thanks

[Georg Pauwen]

Hello,

if the ASBR is in the same routing domain as the ABR, you cold use the 'default-information originate always' command on the ASBR.

Regards,

GP

[dwyerr]

Hello.

Not sure about filtering. You can, however, prefer one ABR over another by increasing the cost of the default route on the ABR you want to be less attractive.

area x default-cost [value]

-Rob

[ruwhite]

If you choose to use default-information originate on the ASBR, you'll need to make certain you set your metrics as Rob notes.... And you won't be able to get the ABR's default preferred over the ABR's unless you use default information origination on the ASBR (since the ASBR's default is probably an external, rather than an internal). You should be able to filter the ABR's default, though using type 3 filtering:

http://www.cisco.com/en/US/products/sw/iosswrel/ps1839/products_feature_guide09186a00800b5d4f.html

Or at least I would think so (I've not run it up in the lab to check it, though).

:-)

Russ.W

[dwyerr]

Question.

Does it really matter that one is an ASBR and one is an ABR ?

If both routers are connected to the stub they are both injecting an "IA 0.0.0.0/0" route by default.(area x default-cost [value] will make one of the 2 IA routes a better choice.)

Now that I think of it, the route generated using "default-information oringate always" on the ASBR will not even make it into the stub area, right ? This is an external route.

-Rob

[zhichao]

hi. thanks for your advise.

tuning the area cost may not work. ASBR generates default routes as external type, but ABR generates it as IA type, which is preferred by the routers (without comparing cost).

OSPF ABR Type 3 LSA Filtering may be a good idea. we will try it.

[Harold Ritter]

From a design perspective, if you have a "totally" stubby NSSA, how do you provide connectivity from this area to the rest of the AS. Usually the default gateway originated by the ABR as a IA is the one route that provides connectivity back to the rest of the AS. Are you really running a NSSA with "no-summary" on the ABR?

[zhichao]

too bad. this feature not supported on Cat6500. :(

Any other idea?

[dwyerr]

Am I missing something,

area x default-cost [value]

does not work ?

The whole purpose of this is to change cost on the default IA route.

-Rob

[zhichao]

yes, the command is to change the IA route cost.

However no matter how much the IA route cost is, it is still preferred than the External route generated by ASBR.

By default, ASBR genenrates default route as external type, and ABR generate IA type.

thansk

[dwyerr]

Well yes, that is sort of my point. The external default route will not make it into the area because it is a stubby area and the default is an External LSA. Even if you filter out the IA route I would think that the default route being advertised by the ASBR would not make it into the area anyways, because it is type 5.

Guess the topology is not clear to me. If you have two routers attached to the stub area, one of them being an ASBR you can cost the links so the ASBR is preferred.

-Rob

[Harold Ritter]

In an NSSA you can generate the default as a type 7 LSA as opposed to a type 5 LSA, which as you said won't be permitted in a stubby or NSSA.

The type 7 LSA will in turn be transLATED into a type 5 by the ABR, which in doing so becomes the ASBR for the external route.

[dwyerr]

Oh well, I am reminded of how difficult it used to be working in support, reading email and trying to find out what someones network looks like is a bit challenging :-)

I defenitely understand the type 7 to type 5 LSA to provide connectivity for an external network attached to a NSSA. Just not sure how this has anything to do with a default route coming from area 0 into the NSSA.

-Rob

[ruwhite]

Hmmm.... I see this committed into 12.1(08a)E, which should be the CAT 6500, right? It's a generic IOS feature, so it should be there someplace, I would think (?).

Russ.W

[Harold Ritter]

Russ,

You are right. It is definitely available in the 12.1E train. I'm running 12.1(8b)E13 and this feature is available.

XB-MSFC-10:8B#sh ver

*Oct 3 10:59:11.271: %SYS-5-CONFIG_I: Configured from console by console

Cisco Internetwork Operating System Software

IOS (tm) MSFC Software (C6MSFC-JSV-M), Version 12.1(8b)E13, EARLY DEPLOYMENT REL

EASE SOFTWARE (fc1)

BXB-MSFC-10:8B(config-router)#area 3 ?

authentication Enable authentication

default-cost Set the summary default-cost of a NSSA/stub area

filter-list Filter networks between OSPF areas

nssa Specify a NSSA area

range Summarize routes matching address/mask (border routers only)

stub Specify a stub area

virtual-link Define a virtual link and its parameters