I broke down and refered to RFC 2138 to see if there
was a description as to why all routers in a stub
area must be configured as stubs. In section 3.6,
there is a two sentence paragraph which provides
a bit of information:
The OSPF protocol ensures that all routers belonging to an area agree
on whether the area has been configured as a stub. This guarantees
that no confusion will arise in the flooding of AS-external-LSAs.
So, the reason for tagging each stub router as being
in the stub is to make sure there is no confusion regarding
the flooding of AS-external LSAs. The Hello protocol does
this enforcement. If a type 5 LSA was to somehow magically
appear in the stub area, each router would know not to flood
it. Ok, I buy that.
Now, the ABRs are going to be the ones that flood AS-external
LSAs into the stub area. It should be sufficient to tag only
the ABRs as stubs (much like only the ABRs are "no-summary"
tagged for Totally stubby areas) to prevent these type 5 LSAs
from getting into the stub. But I'd buy the argument that by
configuring all area stub routers as being stubs will help detect
misconfiguration of an stub ABR.
If the reason is to maintain consistency, then why is
the no-summary not also applied to all internal stub
routers when they're in a Totally stub area?
Not at peace yet...
Brian
-----Original Message-----
From: Thomas Trygar [mailto:[EMAIL PROTECTED]]
Sent: Thursday, May 11, 2000 6:16 AM
To: Field, Brian
Cc: [EMAIL PROTECTED]
Subject: Re: OSPF question- stub area
Brian,
That's the way they made it. Accept it and be at peace.
The ABR will inject a default route X.X.X.X 0.0.0.0 X.X.X.X into area for
other
stub areas. This is so stub area routers know that for anything they don't
know,
go here (second X.X.X.X).
All area routers' Link State Databases must match.
The Hello packet E-bit ('stub area' option bit) must be correct on all area
routers since entire Hello Packet must match for adjacencies to form.
Tom Trygar
"Field, Brian" wrote:
> Thomas,
>
> Thanks for the response.
>
> I'm still unclear as to why an area internal router needs
> to know that it is in a stub area. If the ABRs are tagged
> with the stub indicator and are filtering type 5 LSAs from
> being advertised into the area, why would an internal router
> need to be configured to know this?
>
> Suppose we were to remove the stub flag from the Hello protocol.
> All routers in a stub area would then negotitate the two-way
> state and evetually have the same link database.
>
> How would the lack of a router knowing it was in a stub
> area impact OSPF operation on the router? For a Totally
> stubby area, the internal routers are not aware that they're
> in a TSA and won't be receiving type 3/4 LSAs. So why
> must an internal stub router know that it is in a stub?
>
> Thanks
> Brian
>
> -----Original Message-----
> From: Thomas Trygar [mailto:[EMAIL PROTECTED]]
> Sent: Wednesday, May 10, 2000 9:08 PM
> To: Field, Brian
> Cc: [EMAIL PROTECTED]
> Subject: Re: OSPF question- stub area
>
> Brain,
>
> All routers within a stub area must have identical Link State Databases.
>
> "Field, Brian" wrote:
>
> > When configuring a stub area, why is it that all routers
> > in the stub area must be tagged as being in a stub?
> >
> > Ok, so the Hello mechanism requires that adjacent routers
> > agree that they are in the same area and both have been
> > configured as being in a stub area.
> >
> > Besides the Hello negotiation, what is the reason for requiring
> > this? Why is it not sufficient to tag the stub area ABRs as
> > being stubs and be done with it (much like is done with the
> > "no-summary" tag being required only in ABRs to a totally
> > stubby area)?
> >
> > Thanks
> > Brian
> >
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