ES-IS forms adjacencies between ESs and ISs. ES-IS performs the following tasks in a process known as configuration. Configuration must be completed prior to routing between ESs.
i) Identifies the area prefix to ESs (similar to DHCP in TCP/IP).
ii) Creates adjacencies between ESs and ISs.
iii) Creates data link to network address mappings (similar to ARP in TCP/IP).
The Operation of End System to Intermediate System (ES-IS)
ESs (hosts) send End System Hellos (ESHs) to a well-known multicast addresses to announce their presence to ISs (routers). Routers listen to ESHs to discover the hosts on a segment. Routers include information on ESs in their LSPs to other routers (ISs). ESHs are generated by ESs and are sent to all ISs (L1, L2, and L1/L2) on the subnetwork.
ISO end systems use ESHs to attach to intermediate systems. IP end systems do not send ESH, therefore Integrated IS-IS only attaches the directly connect subnets.
ISs (routers) send Intermediate System Hellos (ISHs) to a well-known multicast addresses to announce their presence to ESs. ESs listen for ISHs and randomly select an IS on its directly attached network to forward their packets to other ESs. ISHs are generated by ISs and are sent to all ESs on the subnetwork.
ISs use IS-IS Hellos (IIHs) to establish and maintain adjacencies (heartbeats) between them. IIHs are transmitted separately at Level 1 and Level 2.
IP hosts do not use ES-IS. IP has its own processes and applications to handle the same functions as ES-IS, eg: Internet Control Message Protocol (ICMP), Address Resolution Protocol (ARP), and Dynamic Host Configuration Protocol (DHCP).
Although Integrated IS-IS is able to support IP exclusively, yet it still uses CLNS to transmit reachability information and still forms adjacencies using ES-IS and IIHs.