Hi Roch,
Agree with your explainatoin - rrSee one additional comment below.
Roch Guerin wrote:
Rick,
Not sure there is a unique explanation, but I would take the statement
with a grain of salt as I know of a couple of implementations that do
things more intelligently and indeed avoid a Dijkstra recomputation
(full SPF) when the change only affects a stub network in an area (in
Section 16 of RFC 2328, the intra-area routing table computation is
actually broken into two steps, with the second step being dedicated
to adding stubs).
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
particular, unless you do a detailed parsing of the RouterLSAs and
NetworkLSAs to determine what has changed, the receipt of a changed
one will often be used as a sufficient trigger to schedule a Dijkstra
computation.
Now this is again a necessary but not a sufficient condition, and some
implementations will perform the additional parsing required to avoid
a full SPF (Dijsktra) when it is not needed.
Exactly, in fact, this is stated explicitly in RFC 2328.
The specification does not require that the above two stage
method be used to calculate the shortest path tree. However, if
another algorithm is used, an identical tree must be produced.
For this reason, it is important to note that links between
transit vertices must be bidirectional in order to be included
in the above tree. It should also be mentioned that more
efficient algorithms exist for calculating the tree; for
example, the incremental SPF algorithm described in [Ref1].
Thanks,
Acee
hope this helps,
roch
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?
thanks
/rg
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