In the event of certain topology changes, for instance non-local adjacency flaps, IS-IS employs incremental SPF optimizations to efficiently update the routing table. An incremental SPF is faster and takes fewer CPU cycles than a full SPF.
IS-IS can be configured to perform a full SPF calculation when any network topology change occurs so that IS-IS always runs full SPF for all such network changes.
Note
If you disable the partial SPF optimizations, IS-IS automatically disables the incremental SPF optimizations and always runs full SPF. However, the reverse is not true: disabling incremental SPF optimizations does not disable partial optimizations.