Chapter 2 Advanced Routing for FortiOS 5.0 : Intermediate System to Intermediate System Protocol (IS-IS) : IS-IS background and concepts
  
IS-IS background and concepts
Intermediate System to Intermediate System Protocol (IS-IS) allows routing of ISO’s OSI protocol stack Connectionless Network Service (CLNS). IS-IS is an Interior Gateway Protocol (IGP) not intended to be used between Autonomous Systems (ASes).
This section contains:
Background
How IS-IS works
Parts and terminology of IS-IS