Why OSPFv3 is more faster than OSPFv2

[Meddane]

The big problem with OSPFv2 is the Router LSA which carries many informations through Link Types 1 2 3 ( Neighboring p2p or transit network, virtual link ) in addition it lists all IP prefixes for links in p2p with Link Type 4. Any changes of Link Types ( 1 2 3 4) triggers Full SPF calculation. OSPFv3 improves SPF calculation by removing IP prefixes from Router LSA and gives the responsability to Type-9 LSA so that only when a router lost its neighbor then full SPF is triggered since it impacts all routes through this neighbor while any change of IP prefixes ( new prefix added or old prefix removed) in the Type-9 LSA will not trigger Full SPF calculation. Changes of IP prefixes occurs more frequently than losing the neighbor router.

Another reason for this evolution is that in OSPFv2 a large Router LSA with many links can be flooded with the risk to exceed the MTU .

Removing the IP prefixes from Router LSA in OSPFv3 reduces the size of this LSA making flooding more optimized.

Removing the IP prefixes from Router LSA in OSPFv3 reduces the size of this LSA making flooding more optimized.

[Martin L]

We appreciate your hard work ! thank You for sharing!

[Joseph W. Doherty]

As a possibly interesting side note, concerning "Any changes of Link Types ( 1 2 3 4) triggers Full SPF calculation.", Cisco introduced Incremental SPF.

In a Cisco OSPF FAQ, we find:

Q. How does ISPF impact or improve the OSPF network?
A.Incremental SPF is more efficient than the full SPF algorithm, which allows OSPF to converge faster on a new routing topology in reaction to a network event. The incremental SPF is designed in such a way that it only updates the affected nodes and does not rebuild the whole tree. This results in a faster convergence and saves CPU cycles because the unaffected nodes do not need to be processed.  With a best practice ISPF would make more of a difference for a large OSPF domain.

Incremental SPF provides greater improvements in convergence time for networks with a high number of nodes and links. Incremental SPF also provides a significant advantage when the changes in the network topology are further away from the root of the SPT; for example, the larger the network the more significant the impact. A segment of 400-1000 nodes must see improvements. However, it can be hard to verify in a deployed production network without some kind of facility or tool to measure the end-to-end delay. For more information, refer toOSPF Incremental SPF.

Yet in this Cisco Change of Default OSPF and IS-IS SPF and Flooding Timers and iSPF Removal TechNote, we find this feature has been removed, as with the more powerful platforms, this feature is no longer is considered worthwhile.

None of the forgoing negates that OSPFv3 may perform SPF faster than OSFPv2 having less OSPF overhead, but possibly, for the same reasons listed for removal of iSPF, the calculation delta between the two OSPF variants might not be practically significant.

The last reference, the TechNote, also describes a new default for Cisco OSPF timers.  In my experience, decreasing those timers, in earlier releases, seemed to be more critical for fast OSPF convergence than the time routers actually took to perform SPF.

Lastly, much of the impact of "sloshing" about voluminous LSAs, might be addressed in OSPF area design and usage.  (Or the opposite, as when I see OSPFv2 routers, willy-nilly, use redistribute connected.)

Again, the forgoing doesn't negate the "improvement" in the OSPFv3 approach, but in large networks, some of the advanced  (and/or proprietary [which Cisco's are an interesting study]) features, of various routing protocols, often are to counter "scaling" issues as the network topology becomes larger.

[Joseph W. Doherty]

"Another reason for this evolution is that in OSPFv2 a large Router LSA with many links can be flooded with the risk to exceed the MTU ."

Indeed, this can be an issue, exceeding the MTU isn't always an issue, but certainly, as with most cases of IP fragmentation, something to be avoided.  For those interested, you may find this OSPF, MTU and LSA Packing Tech Note of interest.