In many cases, the entire SPT need not be recomputed because most of the tree remains unchanged. Incremental SPF (iSPF) 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. Incremental SPF is scheduled in the same way as the full SPF. Routers enabled with incremental SPF and routers not enabled with incremental SPF can function in the same internetwork.
Given only pros and not cons, we should enable iSPF by default. iSPF can be easily enabled using ispf command under each router ospf process.
- router ospf 1
- show ip ospf 1 | inc SPF
- Incremental-SPF enabled