How IS-IS works
As one of the original modern dynamic routing protocols, IS-IS is straightforward. Its routing algorithm is not complex, there are some options to allow fine tuning, and it is straightforward to configure IS-IS on FortiGate units.
The routing algorithm used by the Decision Process is a shortest path first (SPF) algorithm. Instances of the algorithm are run independently and concurrently by all intermediate systems in a routing domain. IntraDomain routing of a PDU occurs on a hop-by-hop basis: that is, the algorithm determines only the next hop, not the complete path, that a data PDU will take to reach its destination.