>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.
Correct.
>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?
Yes. The ASBR, in this case, isn't just "in the stub area."
>In this situation would the router just not
>advertise it's type 4 and 5 LSAs into that particular area?
To be an ABR, a router must connect to the backbone and at least one
nonzero area. Ignoring the case of default information origination,
an ASBR must redistribute something.
The characteristic is of the area, not the ABR. I see no reason why
a physical router might not be connected to:
External source
Area 0.0.0.0
Area 0.0.0.1
Area 0.0.0.2 totally stubby
Area 0.0.0.3 NSSA
Area 0.0.0.4 stubby
It can receive external LSAs from:
its direct connection to an external
area 0.0.0.1
area 0.0.0.3
It will send default (a special type 5) into areas 0.0.0.2, 0.0.0.3,
and 0.0.0.4.
>
>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.
Correct. In any discussion such as this, one has to be very careful
to keep the distinctions clear among:
1. LSDBs
2. Transient routing tables for OSPF (i.e., the output of the
enhanced Dijkstra process, which is sent to the routing table manager
but not otherwise retained)
3. The main routing table
4. Other sources of routing information that are redistributed
into OSPF, and don't necessarily pass through #3 at all.
It's perfectly valid to have "route filters" between #2 into #3, and
from #4 into #1 (which become LSA-5's once into #1).
>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.
Inside an area. I designed a domain for a large company where one
area, which was their communications research lab (one of the
pioneers of the Internet), had its own BGP connectivity. This ASBR
advertised default into the nonzero area. I filtered out the default
route in the direction of area 0.0.0.0.
In like manner, the backbone advertised aggregated corporate routes
into this specific area, but I filtered default from entering the
research lab area from the backbone.
So, the policy enforced was:
General corporate: default to the backbone for other corporate
divisions (in other areas), and for external
destinations
Research: default to their own Internet connection for
external destinations. Go to the backbone for
other corproate destinations.
> >
>>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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