IS-IS supports partial SPF calculation, also referred to as partial route calculation. When an event does not change the topology of the network, IS-IS is not perform full SPF but instead performs an IP reach calculation for the impacted routes. Partial SPF is performed at the receipt of IS-IS LSPs with changes to IP reach TLVs and in general, for any IS-IS LSP TLV and sub-TLV change that does not impact the network topology.