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From: Philip Chen (pchenENNOVATENETWORKS.COM)
Date: Fri May 11 2001 - 09:20:56 CDT
Subrat Mohanty wrote:
> That's what my question
> -----Original Message-----
> From: Mailing List [mailto:OSPFdiscuss.microsoft.com]On Behalf Of Alex
> Sent: Thursday, May 10, 2001 10:24 PM
> To: OSPFdiscuss.microsoft.com
> Subject: Re: An Area problem
> I understand that ASExt Lsa should not be associated with any area. But some
> how I have to keep track of the area info so that while breaking tie
> (16.4.1) I can decide whether the ASExt Lsa is originated from a BB Area or
> a NBB area. Should I consider the area as the area from which I received the
> ASE Lsa or I should consider the area as the area associated with the path
> to the AdvRouter of the ASE Lsa? If it is the second case, then how should I
> determine the area associated with the the ASBR or the route to ASBR?
> Should I examine the Router Lsa of the advertising router?
You do not use area ID from which an ASE LSA is received from for tie breaking.
If the forwarding address for a ASE LSA is ASBR (advertising router) itself, if
you have multiple equal cost intra-area paths to ASBR, you use the one with
largest area ID for tie breaking. This happens when ASBR attached to more than
one common areas as the calculating router.
> If the ASBR lies
> in a different area, then I might not have the Router Lsa. Also I am not
> able examine the route to the ASBR, because by the time I do it I may not
> have the route to the ASBR( Spf Calc might not be done).
You should calculate intra / inter-area routes before you calculate external
> I understand my
> question is pretty implementation specific, so I am trying to see all
> possible scenarios. Can u pl. help me out?
> Regards Subrat
> First off, ASE-LSAs are not associated with a specific
> areas, they are global to the OSPF domain. So, in your
> example below, even if R1 receives the LSA originated
> by R3 from R2 first, the path to the external network
> will be the path to the originating router (or the fwd_addr
> if present) and the path (not the LSA) will be associated
> with the BB.
> Now, if you have more than one ASBR announcing the same
> network, you should follow sections 16.4 and 16.4.1 of
> the spec.
> Alex Zinin
> Thursday, May 10, 2001, 9:05:27 AM, Subrat Mohanty wrote:
> > I have a problem while breaking the tie between routes to the same
> > destination, one thru BB Area and one thru NBB Area. I am explaining the
> > scenario and my understanding as well.
> > +---+ A1 +---+ BBArea +---+
> > + R2+------------+ R1+-----------+ R3+
> > +---+ +---+ +---+
> > A1 = Area 1
> > A2 = BB Area
> > R2 and R3 are ASBR.
> > Let's add a static route 22.214.171.124/8 at R3. The ASExt Lsa will be flooded
> > R1 and from R1 to R2. So all the 3 routers will be in Sync. Let's
> > R1 and restart. If R1 starts establishing adjacancy with R2 first, then R2
> > gives all his Lsa's in it's DDP including the ASExt Lsa for 126.96.36.199 . R1
> > will Request for 188.8.131.52 to R2 and get the ASExt LSA. Then adjacancy with
> > R3 is being established. R3 will also mention the ASExt LSA 184.108.40.206 . But
> > R1 will not request for the same because it has the same copy in it's
> > database. Although the routes will be there in each guys database, the
> > association for 220.127.116.11 in R1 is wrong. Because it learns the route from
> > A1.
> > The problem comes when I have static route 18.104.22.168 in R2 as well as R3.
> > that case, R1 should prefer the route thru R1 regardless of the cost of
> > routes. If I restart R1, then if I get both the ASExtLsa from any one of
> > router R2 or R3, the area associated with the ASExtLsa will be same. So it
> > starts looking at the cost. What stratagy an iplementer should follow?
> > Should he check for the Router LSA (if the adv router is in that area)or
> > ASBR Lsa (the ABR advertising the ASBR is in that area) for the AdvRouter
> > the ASExt Lsa 22.214.171.124 before accepting the ASExtLsa ? Or what else?
> > Regards Subrat