Re: Incrematal SPF (ATTN: Narbik and Marko, et al)

To answer your question very below:

I think that is called a "partial SPF" computation. This occurs
whenever there is a change in LSA Type 3 or Type 5 .. basically a
change outside the area where this partial computation happens. It is
enabled (it is not configurable I think) and it differs from the ISPF.

Incremental SPF is an optional mechanism to make a computation more
efficient whenever there is a change in the area (literally in LSA
Type 1 or 2). It works by modifying only some branches of the shortest
path tree which still belong to the area where the SPT is being
recomputed. When ISPF is in use you don't need to build those parts of
a tree which are not impacted by the change (a cost change on the very
far router in the area, or when stub links flap).

This is how I understand the difference from the OSPF Design Solutions
Guide by Cisco Press.

OdoslanC) z iPhone

DEa Mar 25, 2010, o 4:48 PM, olugbenga lasisi <logpoet_at_gmail.com>
napC-sal:

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> study and even real life challenges in this field . I just want you
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> .....................
>
> Enough on the story ...
>
> I said i was going to post some clarification questions by COB...
> but i just
> came across this some minutes ago and i thought i should ask.
>
>
> OSPF uses Dijkstra's SPF algorithm to compute the shortest path tree
> (SPT).
> During the computation of the SPT, the shortest path to each node is
> discovered. The topology tree is used to populate the routing table
> with
> routes to IP networks. When changes to a Type-1 or Type-2 link-state
> advertisement (LSA) occur in an area, the entire SPT is recomputed.
> In many
> cases, the entire SPT need not be recomputed because most of the tree
> remains unchanged. Incremental SPF allows the system to recompute
> only the
> affected part of the tree. Recomputing only a portion of the tree
> rather
> than the entire tree results in faster OSPF convergence and saves CPU
> resources. Note that if the change to a Type-1 or Type-2 LSA occurs
> in the
> calculating router itself, then the full SPT is performed.
>
> *From the areas i highlighted, my understanding of how ISPF work is
> that it
> is only usefull when you have mutiple areas and there are changes to
> the
> LSA's outside the calculating router's area... Is this correct?*
>
>
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Received on Thu Mar 25 2010 - 18:38:23 ART