ODPF - How to not propagate default gateway between areas?

Maybe I'm not understanding correctly, but can't you just avoid using "default-information originate"?

Do you mean to configuring default gw on each router as alternative? I would like to avoid this.

Is there another method to propagate information about default gateway to routers in area?

Ok, so IIUC the internet-connected routers generate the default routes, and you'd like to block them at the ABR. Topology:

internet----r1a0---(area0 routers)---ABR---(area1 routers)---r1a1---internet

IIUC, r1a0 and r1a1 originate a default route, which is meant to serve as the default route for the routers of the area.

1) filter-list only filters type 3 LSAs, so in this case it does nothing to default routes. 2) if you are worried about having multiple default routes, then have the ASBRs originate the default route using metric type 1. This way, routers internal to an area will use the "closest" default route (which will probably be the one originated by the area's ASBR; if this is not the case, you'll probably be able to adjust the metrics to make this happen). 3) even if you use the default metric type 2, routers internal to an area will still prefer the default route advertised by the closest ASBR, so the same concepts expressed in 2) apply, but this time you have to adjust the metric *to the ASBR*, not the metric of the default route. 4) you didn't say anything about what default route should be used by the ABR. If you don't want it to load-balance between the two (and you probably don't), then make sure its metrics to the two ASBR are not equal, or that it receives the default routes with different metrics. 5) external LSAs are flooded throughout the whole OSPF domain by definition (unless stub or nssa areas are used, but in your case one of the areas is area 0 so this is not an option). If you still think that you want to filter external LSAs, you might run two different OSPF processes on the ABR, and do mutual route redistribution between them, filtering out the unwanted default routes using a route map. No need to say that this solution is awkward to say the least, making all the routes coming from the other "area" look external. If you need to do that, it's usually a symptom of bad network design, so you better reorganize your network in a different way.

a default gateway would have to be an external route - you explain further you have it as an ext2.

So - silly Q - what happens when a DG goes away? it would be common practice to want the out of area one to propagate...

anyhow - to answer.

the only things that you can easily filter in the OSPF database as it propagates between routers are internal routes at an ABR. So you cannot filter it directly.

If you run 2 different OSPF AS, 1 in each area, and run 2 OSPF processes on the ex ABR, you should be able to filter. Bit messy though....

However - you can filter routes from OSPF into the local routing table, so you should be able to "kill" every default route that you dont want in the router - but you would need a policy on every OSPF router (or evey router that isnt in a stub area). that makes this doable, but "delicate" since routers rolled out with the wrong config will not be obviously wrong - ie stuff will work.

This is one i havent done for a long time, so you are going to need to build your own policy...

Note this may not be a problem, since

1. filtering only matters where the 2 defaults point to different next hops.
2. each router picks the lowest cost path across OSPF to any default if it has 2 or more to choose from.

So - if you have the freedom to put the interconnect to the default routes in a suitable place, you may need no filters, or only filters on a couple of routers.

Having said that this sounds like a "tail wagging dog" problem.

1 of the big strengths of OSPF is it is fairly difficult to break a design as the LSDB propagation rules make you treat the network as a single AS. It sounds like you really have a "2 AS" type design issue, and that a single OSPF AS is not the best way to do this.

Hello,

From my perspective you only want/need that ABR will not propagate=20 default route to Area 1. Is it correct? You can achive this configuring Area 1 as a NSSA area. If you configure=20 Area 1 as normal or stub area, ABR will automatically propagate default=20 route!. In OSPF you cannot simply filter it! Consider this.

Regards Ern

Sailor napisa=B3(a):