Routers perform IS-IS routing as follows:
Hello PDUs are sent to the IS-IS-enabled interfaces to discover neighbors and establish adjacencies.
IS-IS neighbor relationships are formed if the hello PDUs contain information that meets the criteria for forming an adjacency.
Routers can build a link-state PDU based upon their local interfaces that are configured for IS-IS and prefixes learned from other adjacent routers.
Routers flood LSPs to the adjacent neighbors except the neighbor from which they received the same LSP. The link-state database is constructed from these LSPs.
A Shortest Path Tree (SPT) is calculated by each IS, and from this SPT the routing table is built.