advertise OSPF default route into a specific area?

[Ondrej Zajicek]

On Sat, Feb 22, 2025 at 03:13:28PM +0100, Maria Matejka via Bird-users wrote:
> Hello Lexi,
> 
> On Sat, Feb 22, 2025 at 01:34:51PM +0000, Lexi Winter wrote:
> 
> > i have an OSPF ABR in area 0 and area 1.  this router has a default
> > route via a non-OSPF interface (kernel route, imported into BIRD using
> > the 'kernel' protocol).  i'd like to advertise this route into area 1,
> > but *not* into area 0.
> > 
> > how would i go about doing that?
> 
> I would filter out the default route on all the area 0 routers.
> 
> > please feel free to let me know if this is a bad idea or if i'm using
> > OSPF wrong :-)
> 
> Well, OSPF isn't intended for filtering routes between areas this way.
> Guessing from what you have written, it may make sense e.g. to run one
> OSPF instance per area, assign each area a BGP private ASN and run BGP
> between them.

If the area 1 is NSSA, it is possible to filter out external routes that
are propagated from NSSA area to backbone using 'external' option:

https://bird.network.cz/?get_doc&v=20&f=bird-6.html#ospf-external

It works in a similar way how 'networks' option can be used to filter
internal routes:

https://bird.network.cz/?get_doc&v=20&f=bird-6.html#ospf-networks

It could be interesting to replace / augment this with BIRD filter
language.

The issue here is that if the external route is originated on ABR, it
could either generate NSSA-LSA in the NSSA area, or just regular Ext-LSA
in backbone area. That is something that perhaps should be configurable,
but i think we always generate Ext-LSA on ABRs.

-- 
Elen sila lumenn' omentielvo

Ondrej 'Santiago' Zajicek (email: santiago at crfreenet.org)
"To err is human -- to blame it on a computer is even more so."