why OSPF does full SPF Calculation for LSA 1 and 2 ??

[mohammed hashim]

hi,

I am reading about OSPF tuning and found a feature called Incremental SPF (iSPF) !!

and briefly this feature enable OSPF router to partially run SPF calculation for LSAs type 1 and 2 (Intra Area), which is without this feature, the OSPF router will do full SPF calculation for these events.

my question is why from the beginning partial calculation was not implemented for this case ?

is there a technical concern in this regard? or it is just a thought of the protocol developer ?

thanks, 

[Richard Burts · Accepted solution]

Receipt of LSA type 1 or 2 indicates that within the area there has been a change in topology. A basic premise of OSPF is that each router must have an accurate understanding of the complete topology within that area. So a change in topology (LSA type 1 or 2) would trigger the SPF calculation and the calculation processed the entire Shortest Path Tree. As we gained experience with OSPF and as our networks grew larger and more complex there was a realization that a topology change on one neighbor did not necessarily change the topology in other parts of the tree and so the complete SPF calculation was somewhat redundant. Implementation of incremental SPF calculation reduced the calculation to the parts of the Shortest Path Tree that were actually impacted by the reported change and leaves the rest of the tree as previously calculated.

As for why this feature was not included in the original specification of OSPF there are multiple factors which include Incremental SPF calculation requires that OSPF maintain additional information about the tree so that it can accurately identify which parts of the tree are impacted and need calculation. Early implementations of OSPF chose not to maintain this extra information in attempts to simplify their processing. Also the networks for which the original OSPF was designed were not as large or complex and the benefits of incremental SPF were not as significant.

HTH

Rick

[Mark Malone]

sorry Richard didn't see you were replying to that already had the page open

[Richard Burts]

Mark

No problem. I think that usually it is a benefit when multiple people respond to a question. In this case we both share the perspective that the Incremental SPF became more desirable as scale and complexity of the network increased.

HTH

Rick

[mohammed hashim]

thanks a lot Richard and Mark for answering this question, it is clear,

if you do not mind I have another but simple question,

does full SPF calculation means the OSPF router will run SPF again against all prefixes within the same area ?

[Richard Burts]

I am glad that our explanations have helped you to understand about Incremental SPF. In a full SPF calculation the OSPF process uses every one of the LSA type 1 and 2 that it has received in that area and starts from scratch in building the Shortest Path Tree. After calculating using all of the type 1 and 2 LSA it has built the SPF Tree and uses it to find the shortest path to each destination and uses them to build the routes for the routing table.

In an incremental SPF the OSPF process remembers what it has built in the SPF Tree and uses the type 1 and/or 2 LSA that it received to determine which part of the tree has changed. It then uses the LSA to calculate the new part of the tree that needs to be updated but does not calculate the other parts of the tree.

HTH

Rick 

[Mark Malone]

Hi

My understanding of it it was introduced later on as networks got larger and scalability/convergence times became an issue as more sensitive time critical services were introduced and the likes of distance vector protocols like eigrp with extremely fast convergence came into play,as ospf is link state everyone needs to know what each other knows so the ripple effect of having to update networks/areas with more than a couple of hundred routers and communicating a full table to each device takes time and uses up large resources , an area is bound by memory to the smallest router , if an 800 series can only have enough mem to hold few thousand routes the 3945 can only have the same so reducing the use of resources in a general area is very important as not all devices may be equal in specs , the iospf feature prevents unnecessary floods of traffic throughout the network which is obviously important.OSPF dates back to the 80s so as it moves along enhancements are added to the protocol like everything else to keep it up to date and useful in present networks