Routers using IS-IS issue Hello packets out of all interfaces enabled for IS-IS to discover neighbours and establish adjacencies.
Routers will become IS-IS neighbours if their Hello packets contain certin information for the creation of an adjacency.
Routers then build Link-State Packets (LSP's) based on the adjacencies present and then flodd the LSP's to their neighbours.
All routers then build a Link-State Database (LSDB) using the information in the received LSP.
The router then runs the SPF algorithm, the shortest path to each dsestination is then calcultated and added to the Routing Information Base (RIB or OSI routing table).
Next, the Partial Route Calcultion is performed using the IP information contained within one of the Time, Length, Value (TLV) fields in the LSP. This generates shortest paths for IP traffic and in turn is entered in to the IP routing table.