That's kindof the conclusion I have come across too, that OSPF just isn't
designed to be able to do this selectively. Obviously blocking LSA type
1's or 2's within an area would directly violate the RFC since ever
Router's link state database in a certain area has to match, and given that
you redistribute into the OSPF process itself and not into a given area it
makes sense that barring certain exceptions such as stub areas an ASBR will
always send the LSA 5's into every connected area.
Here's a slightly different question though for anyone. Could an ABR that
is connected to a stub area (not an NSSA area) be an ASBR? In other words,
does the fact that the router is a backbone router and possibly connected
to other areas as well make it exempt from the rule that you can have no
ASBRs in a stub area? In this situation would the router just not
advertise it's type 4 and 5 LSAs into that particular area?
At 11:52 AM 2/2/01 -0500, you wrote:
>So you are talking about a topology where you have an ASBR that also
>borders Area 0 and is thus also an ABR by definition.
>
>The question is then, "can I control which external prefixes enter the
>rest of the ospf domain as type 5 LSA's" I would have to say that you
>cannot by definition restrict the flow of LSA's in an OSPF domain beyond
>turning down interfaces. You can decide which area's will recieve Type
>3/4/5/7/10 etc by the use of stub areas and their variants and you can
>also minimize prefix flow's by summarizing at borders. However, there is
>no way to my knowledge that you can instruct a ABR to advertise some but
>not all type 5's into an area.
>
>Route filtering in OSPF is somewhat of a misnomer in my opinion. Since
>link state protocols do not advertise their routing table, but instead
>their link state database, the concept of prefix filtering within an OSPF
>domain is out of place. In fact, filting LSA's of any kind within an area
>directly violates the RFC in that all routers in an Area MUST maintain
>identical copies of the area topology. However, filtering type 5's at the
>ABR, to me, has some merits. I may be missing something here and Howard
>will likely point it out if I am. But in summary, there is no
>"distribute-list" type command that allows you to restrict the flow of
>LSA's within an OSPF domain.
>
>Naturally I'm sure your aware that you can filter prefixes like mad when
>dealing with redistribution into or out of OSPF.
>
>HTH
>
>-pete
>
>
>*********** REPLY SEPARATOR ***********
>
>On 2/2/2001 at 7:48 AM Curtis Call wrote:
>
> >Anything, but to make it simple lets just say we're trying to redistribute
> >static routes.
> >
> >At 08:41 AM 2/2/01 -0500, you wrote:
> >>Redistribute what?
> >>
> >>
> >>*********** REPLY SEPARATOR ***********
> >>
> >>On 2/1/2001 at 8:02 PM Curtis Call wrote:
> >>
> >> >There's something that I'm curious about dealing with OSPF ASBRs. Let's
> >> >say your ASBR is also an ABR that is bordering area 0 and area 1. Is
> there
> >> >a way that you could specify to only redistribute into area 1 or by
> nature
> >> >of being an ASBR does a router have to advertise the route to every
> area to
> >> >which it is connected (assuming all areas are normal non-stub areas).
> >> >
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