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

Miroslav and Joe,

Thank you so much , you guys have been very helpfull. I now have a good
understanding of the concepts.. Here is a summary of my understanding.

Partial SPF.

This happens when there is a change to LSA type 3,4,5,7(any change outside
the area)

ISPF

Occurs when there are changes to LSA types 1 and 2 far away from the root
router (The calculating router)

Full SPF.

Will compulsorily happen if there is LSA type 1 and 2 changes in the root
router.

Gbenga.
On Thu, Mar 25, 2010 at 7:01 PM, Joe Astorino <jastorino_at_ipexpert.com>wrote:

> To answer your other question:
>
> -- iSPF and Partial SPF are two different things
> -- iSPF is incremental SPF and allows the SPF algorithm to only have
> to run in places where things have changed instead of running on the
> entire SPF algorithm on every router in the area. It is much more
> efficient
>
> -- Partial SPF, also known as Partial Route Calculation (PRC) is a bit
> different. My understanding is that PRC is specifically for IP
> prefixes. In other words, when an IP address/mask is changed, it
> prevents SPF from having to be run fully on all the routers -- There
> is no need to do this when an IP prefix is changed, at least for type
> 3, 5 and 7 LSAs is OSPFv2.
>
> In summary, the SPF tree is something independent of the IP prefix
> information actually being communicated. iSPF deals more with the SPF
> tree being calculated only in necessary places when the tree changes.
> PRC deals with partial recalculations needed when only the IP prefix
> information being communicated changes.
>
> HTH!
>
>
>
> On Thu, Mar 25, 2010 at 6:16 PM, <jastorino_at_ipexpert.com> wrote:
> > 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.
> >
> >
> > Sent from my Verizon Wireless BlackBerry
> >
> > Regards,
> >
> >
> >
> > Joe Astorino - CCIE #24347 (Earned not bought)
> > Sr. Technical Instructor - IPexpert
> > Mailto: jastorino_at_ipexpert.com
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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*
> >>> The goal of this is not to stimulate rivalry or competiton between the
> >>> parties addressed. Rather it is an effort by a soldier that has been
> >>> fallen
> >>> twice but not detered, a soldier whose sole goal is the medal of honor
> >>> that
> >>> awaits him at the finish line. One who wants to take advantage of the
> >>> battle
> >>> strategies, knowlege and wisdom of generals who have trod this path
> >>> before.
> >>>
> >>> I have the decided to engage them, 'cos since i have been following
> this
> >>> group they have been very consistent at sharing great insights on
> members'
> >>> study and even real life challenges in this field . I just want you
> guys
> >>> to
> >>> make a general out of me, be part of my success story.
> >>> .....................
> >>>
> >>> 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?*
> >>>
> >>>
> >>> Blogs and organic groups at http://www.ccie.net
> >>>
> >>> _______________________________________________________________________
> >>>
> >>
> >>
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> >
> >
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> >
> > _______________________________________________________________________
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> >
> >
> >
> >
> >
> >
> >
> >
>
>
>
> --
> Regards,
>
>
>
> Joe Astorino - CCIE #24347 (Earned not bought)
> Sr. Technical Instructor - IPexpert
> Mailto: jastorino_at_ipexpert.com
> Telephone: +1.810.326.1444
> Live Assistance, Please visit: www.ipexpert.com/chat
> eFax: +1.810.454.0130
>
> IPexpert is a premier provider of Self-Study Workbooks, Video on
> Demand, Audio Tools, Online Hardware Rental and Classroom Training for
> the Cisco CCIE (R&S, Voice, Security & Service Provider)
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Received on Thu Mar 25 2010 - 20:43:01 ART