К тому же, оно deprecated и вполне вероятно уже удалено.
«iSPF is Removed
Reason for the Change
Incremental SPF is a feature which was developed circa 2000 as a means to optimize performance of the IGP, that reduces the execution time of an SPF by only recalculating a sub-tree of the full Shortest Path Tree when topology changes were limited to a portion of the network. Implementation of this feature has been superceded with ever increasing CPU power on routers, which meant that when you use iSPF, it became less and less necessary, because of providing almost no benefit, while adding complexity. For this reason, it is generally recommended to not enable this feature, because with higher CPU power the use of regular SPF is more straightforward. This is also why Cisco chose to deprecate the iSPF feature. The CLI that exists is still accepted but does not enable the feature and the command will not be NVGEN'ed. A warning message is printed that indicates that the feature is no longer supported.»