Intermediate System to Intermediate System

Intermediate System to Intermediate System(invariably known as IS-IS) is a routing protocol; initially designed for the ISO protocol suite ('Intermediate System' is their term for a router), it has been adapted for use with the TCP/IP protocol suite.

It is a link state routing protocol; it was mostly inspired by the first LS routing protocol, that used on the ARPANET, but includes a few improvements to fix issues found on the ARPANET.