Re: NSSA and Totally NSSA

From the network design viewpoint, NSSA areas were introduced to "contain" a
set of external routing information received from a different routing domain
within a single OSPF area. Normal NSSA areas allow receiving an external
*default route* and possible bridging it to the rest of the local OSPF
domain.

On contrary, totally NSSA areas MUST use the default route toward the local
backbone, as summary LSAs (local OSPF domain routes) are blocked from
entering the totally NSSA area, and there is no other way to reach them.
This makes it impossible to import external default route with totally
NSSAs, as type 3 routing information always overrides type 7, but allows for
reducing the amount of routing information in NSSA area.

Also, this is why the NSSA ABR only injects type-3 default route when you
issue the "no-summary" option - with normal NSSA, it gives a chance for NSSA
ASBR to inject its own, type-7 default route.

To sum it up, you would use normal NSSA if you want the area to contain
external default route or maintain accurate routing information for other
local OSPF areas. Otherwise, totally NSSA would almost always be a better
choice due to reduced amount of routing information.

-- 
Petr Lapukhov, petr_at_INE.com
CCIE #16379 (R&S/Security/SP/Voice)
CCDE #20100007
Internetwork Expert, Inc.
http://www.INE.com <http://www.ine.com/>
Toll Free: 877-224-8987
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2011/3/18 Ronnie Angello <ronnie.angello_at_gmail.com>
> That's true...  the real difference with 'totally' is that it also stops
> type 3s from being flooded into the area.
>
> On Mar 18, 2011 12:18 PM, "me you" <anunda19_at_gmail.com> wrote:
>
> I though all stubby areas stopped type 4 and type 5 LSA's.
>
>
>
>
> On Fri, Mar 18, 2011 at 7:39 PM, Aaron <aaron1_at_gvtc.com> wrote:
>
> > Ahh thanks garry, I see Abr in...
>
>
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