Explain

[Tareq Emad Al Asfoor]

Hello , 

Any one can explain to me what we mean by ISPF  with ospf .. Actually i read it in them articles but i didn't got it .. it wasn't clear to me .. any one have clear explain .. Thanks in advance 

[Mark Malone]

Hi

right so think how does ospf work , its link state protocol so when there's a change the whole area that contains all ospf routers gets the change and the whole topology is sent to each individual router

Might not seem to bad and pretty smart now everyone has the same table same routes etc but if you have 200 routers in your ospf domain and 1 small change is made you are using a huge amount of resources across the network just to tell everyone about this change so they came up with ISPF basically instead of sending the whole table it gives  less intensive incremental opsf update saving on resources cpu/memory etc

easily applied under the process itself

router ospf 100

ispf

[Tareq Emad Al Asfoor]

Thanks ..  As you said .. if we have 200 node and a small change happen .. Here by using ISPF is the update will through to all 200 nodes ? Thanks in advance  

[Mark Malone]

The change will be pushed but not the whole topology table to each router like standard ospf updates

[Joseph W. Doherty]

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Posting

Do you understand how OSPF's SPF computes intra-area best paths?  If so, the difference is, as Cisco notes:

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

What are you struggling with that's unclear?

In principle, it's like having a list of numbers that are summed.  If you change just one number, you would need to sum the whole list, unless you knew the difference between the original number and the replacement number.  In that case, you would only need to also apply the difference to the prior total.  Much less "work", faster result.