Subject: RE: [OSPF] intra area prefix lsa change requires spf

From: "Naidu, Venkata" <[email-masked]>
Date: Fri, 11 Aug 2006 14:05:55 -0400

Roch,
 
The question is about OSPFv3 (RFC2740) not about OSPFv2 (RFC2328). OSPFv3
separates topology information from reachability information just like ISIS.
 
I think, the answer to the question is: NO, change to a prefix LSA doesn't
need SPF (neither full nor partial).  If you arranged your data structures
to reflect
last SPT, simply locate the node and then update (add/modify/delete) routes
using the new prefix LSA. This is not an optimization to SPF. I would say an
implementation doing full SPF when a prefix LSA is changed is wasting time
because it would get the same SPT anyway. Just because post SPF work
(updating FIB etc) is time consuming, wasting time in doing unnecessary
(full SPF) is not a good implementation either.
 
Venkata.

I think the origin of this statement is that unlike IS-IS where because
reachability information is encoded into separate TLVs there is a clean
demarcation between running the (full) SPF (Dijkstra) using routers,
pseudo-nodes and links connecting them, and what is called the partial route
computation (PRC) that involves only the prefixes and not the topology, the
situation is a bit murkier in OSPF.  

In section 3.5.3, RFC2740 indicates the the entire routing table is
recalculated when there's a change to the router,network, intra-area and
link LSAs.  Why is it that a change to an intra area prefix would require a
full SPF?

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