>In nssa, when an nssa border router elected as translator loses
>it's translator status. after the translator stability interval has
>expired, rfc suggests to flush only the aggregated translated lsas
>but not the normal translated type-5 lsas. why give special treatment
>to only aggregated lsas any specific reason.
The last sentence in NSSA RFC 3101 Section 3.3 states
...This minimizes the flushing and flooding impact on the transit
topology of an NSSA that changes its translators frequently.
At the time I wrote this I was reflecting on some painful experiences with
router flap. A continuous flap of an NSSA's elected translator might have
an adverse effect on its processing capability as well as the full OSPF
transit topology. Since these Type-5 translations simply mirror their
corresponding Type-7 advertisements, allowing them to simply age out seemed
appropriate. If a translator is re-elected shortly thereafter it could then
re-assume its translator duties without re-originating these Type-5
translations. Of course for this statement to be fully effective an
implementer must mitigate the effect on Type-5 translations of the
following note from OSPF RFC 2328 Section 12.4.4.1:
...if two routers, both reachable from one another, originate
functionally equivalent AS-external-LSAs (i.e., same destination,
cost and non-zero forwarding address), then the LSA originated by
the router having the highest OSPF Router ID is used. The router
having the lower OSPF Router ID can then flush its LSA.
A similar statement is included for Type-7 LSAs in RFC 3101 Section 2.4.
Even if this statement is not mitigated, a delay by the deposed translator
in flushing Type-5 translations until the new NSSA translator's Type-5
translations are receive seems beneficial. Premature flushing effects
routing.
Note that at the time NSSA RFC 3101 was published routing engine processing
power and memory was not what it is today; also bandwidth was expensive so
that OSPF deployments over much slower links than are currently used was
commonplace. Given the kinds of memory that modern routers have perhaps the
last sentence in the above note from 12.4.4.1 is not even necessary. It is
certainly not a requirement for inter-operability.
Pat
Date: Tue, 11 Jan 2011 20:38:30 +0530
From: p6 c6d6 <[email protected]>
Subject: [OSPF] flush translated lsas
Sender: [email protected]
To: [email protected]
hi,
In nssa, when an nssa border router elected as translator loses
it's translator status. after the translator stability interval has
expired, rfc suggests to flush only the aggregated translated lsas
but not the normal translated type-5 lsas. why give special treatment
to only aggregated lsas any specific reason.
thanks
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