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### Instruction:
What is the purpose of the ispf command in IS-IS?
### Response:
The ispf command is used to enable Incremental SPF (iSPF) computation. iSPF allows the router to recompute only the affected part of the shortest path tree when a topology change occurs, instead of recomputing the entire tree. This can significantly reduce the CPU load and improve convergence time in large networks.
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