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

I believe your answer is pretty well laid out in previous post in this thread. iSPF is incremental...more efficient. It prevents every router from having to run the entire algorithm during certain changes.

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Regards,

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-----Original Message-----
From: olugbenga lasisi <logpoet_at_gmail.com>
Date: Thu, 25 Mar 2010 17:43:08
To: Miroslav Kosut<miroslav.kosut_at_gmail.com>
Cc: GS CCIE-Lab<ccielab_at_groupstudy.com>
Subject: 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*
>>
>> *Disclaimer*
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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 - 22:16:56 ART