Subject: Examining Transit Area's Summary-LSAs

From: [email-masked]
Date: Mon, 27 Oct 2003 02:46:58 -0600

I have some doubt about bullet 5 of section 16.3 in 2328.
It says 
=========
If this cost is less than the cost occurring in N's routing
table entry, overwrite N's list of next hops with those used
for BR, and set N's routing table cost to IAC. Else, if IAC
is the same as N's current cost, add BR's list of next hops
to N's list of next hops. In any case, the area associated
with N's routing table entry must remain the backbone area,
and the path type (either intra-area or inter-area) must
also remain the same.
=========

and Section 16.1.1 says:
=======
If the destination is a router connected to the calculating 
router via a virtual link, the setting of the next hop 
should be deferred until the calculation in Section 16.3.
=======

Let say a router that is one end of the virtual link,
learns about a prefix N with cost 4 while calculating 
inter-area routes. It installs a route with no nexthop
because the nexthop destination is connected via a virtual
link. Now when it starts examining the transit area summary
LSAs, it finds an LSA describing prefix N with cost 3 and
the virtual link end point is 2 hops away. So the iac 
becomes 5. Now as iac is more than the previously calculated
cost 4, no nexthop is installed according to algorithm
described above. The route remains without the nexthop and 
thus unusable.

Am I missing something ..

I have a setup where the above scenario becomes a reality
and a route becomes unusable :( I have fixed the problem
by extending the algorithm described above.

=====
If iac is more than the cost occurring in N's routing
table entry AND there is no nexthop associated with N's
routing table entry, then add BR's list of next hops
to N's list of next hops and set N's routing table cost 
to IAC.
=====

Comments/Questions ??

Regards
Mukesh

---

From: Acee Lindem <[email-masked]>
Date: Mon, 27 Oct 2003 09:50:54 -0500

Hi Mukesh,

[email-masked] wrote:

>inter-area routes.
I believe your scenario could also apply to intra-area backbone prefixes.

>It installs a route with no nexthop
I guess the assumption is that ABR that is the virtual link endpoint
will always
advertise a summary into the transit area which has same cost as the
backbone
path.

>to N's list of next hops and set N's routing table cost
I must admit that I don't defer the next hop calculation until
processing transit area
summaries. Rather, I update the next hops if there is a lower cost path via
transit area.