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

Miroslav,

 I am little confused what's the difference between partial SPF and ISPF? My
thought is that they are the same.

Gbenga.

On Thu, Mar 25, 2010 at 1:38 PM, Miroslav Kosut
<miroslav.kosut_at_gmail.com>wrote:

> 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.
>
> Odoslani z iPhone
>
> Dra Mar 25, 2010, o 4:48 PM, olugbenga lasisi <logpoet_at_gmail.com> napmsal:
>
> *NB*
>>
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>>
>> 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 - 17:43:08 ART