From a high level, IS-IS operates as follows:
● Routers running IS-IS will send hello packets out all IS-IS-enabled interfaces to discover neighbors and establish adjacencies.
● Routers sharing a common data link will become IS-IS neighbors if their hello packets contain information that meets the criteria for forming an adjacency. The criteria differ slightly depending on the type of media being used (p2p or broadcast). The main criteria are matching authentication, IS-type and MTU size.
● Routers may build a link-state packet (LSP) based upon their local interfaces that are configured for IS-IS and prefixes learned from other adjacent routers.
● Generally, routers flood LSPs to all adjacent neighbors except the neighbor from which they received the same LSP. However, there are different forms of flooding and also a number of scenarios in which the flooding operation may differ.
● All routers will construct their link-state database from these LSPs.
● A shortest -path tree (SPT) is calculated by each IS, and from this SPT the routing table is built.
Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 4288-4303). . Kindle Edition.