Subject: On detecting change in TransitCapability of an area

From: "Sudharsana V - CTD, Chennai." <[email-masked]>
Date: Thu, 19 Sep 2002 11:21:39 +0530

Hi,
When TransitCapability of an area (say a1) changes, the Backbone prefixes
need to be re-aggregated or de-aggregated in to it.
On doing so, should we flush the summary LSAs describing backbone prefixes
that were earlier(before change in TransitCapability) flooded in to a1?
(if we do not flush the old LSAs, they will be in a1's database for MaxAge).

In the case where a1 becomes transitCapable,
We will de-aggregate and the LSAs with more specific prefixes will be in use
in a1, despite the presence of old aggregated LSAs in a1's database for
MaxAge.

In the case where a1 loses transit capability,
We will re-aggregate - But the area a1 will not have the benefit of
aggregation for MaxAge because of the presence of old LSAs for specific
prefixes.

I do not see any other side effect. Are there any other implications that I
am missing?
Regards,
Sudharsana.V

---

From: Acee Lindem <[email-masked]>
Date: Fri, 20 Sep 2002 01:17:29 -0400

> I do not see any other side effect. Are there any other implications that I

Sudharsana,

In practice, you probabably won't run into many problems with this approach
since I'd guess the combination of virtual links and backbone summarization
is not all that common.

However, did you consider:

  1. If you implement the demand circuit DoNotAge capability, the residual LSAs
     could stay around indefinitely (or at least until a neighbor adjacency is
     lost and the LSA is flooded back to you).

  2. When there are other OSPF implementations acting as ABRs for the transit
     area, some will leave the residual LSAs and others will purge them.
     Potentially, this could cause a routing loop for destinations within the
     range which do not have a less specific de-aggregated route.

  3. You'll need to purge residual summary LSAs if your route
     goes away or changes anyway (since it is no longer correct). This
     complexity in itself seems to be motivation enough to do it correctly
     and purge the residual LSAs.

Thanks,
--
Acee