End Systems and Intermediate Systems

In IS-IS, two different hello-type protocols are used to build adjacencies and exchange information—ES-IS and IS-IS. The End System-to-Intermediate System (ES-IS) protocol is used by routers to discover hosts (and hosts to discover routers) and for exchanging configuration information and redirecting packets to a better path.

The IS-IS protocol builds and maintains adjacencies between routers (intermediate systems). This is similar in function to the Hello protocol used in OSPF to discover and maintain neighbor adjacencies.