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

Gbenga,
from your summary, I would say you have a couple of concepts not very clear.

First, there is no such thing as a "root router" in OSPF.
The whole idea of link state protocols is that all routers have all
information to do the routing. It is state info that is transmitted, not
routing info. And, given that all routers use the same logic to derive
the routes, the routing converges.
A consequence of this is that every time there is a state change, either
all or none of the routers in an area have to recompute things.

-Carlos

olugbenga lasisi @ 25/03/2010 21:43 -0300 dixit:
> 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
>>> Telephone: +1.810.326.1444
>>> Live Assistance, Please visit: www.ipexpert.com/chat
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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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>>
>>
>> --
>> 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)
>> certification(s) with training locations throughout the United States,
>> Europe, South Asia and Australia. Be sure to visit our online
>> communities at www.ipexpert.com/communities and our public website at
>> www.ipexpert.com
>
>
> Blogs and organic groups at http://www.ccie.net
>
> _______________________________________________________________________
> Subscription information may be found at:
> http://www.groupstudy.com/list/CCIELab.html
>
>
>
>
>
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>

-- 
Carlos G Mendioroz  <tron_at_huapi.ba.ar>  LW7 EQI  Argentina
Blogs and organic groups at http://www.ccie.net