Understanding ES-IS for CLNS
End System-to-Intermediate System (ES-IS) is a protocol that resolves Layer 3 ISO network service access points (NSAP) to Layer 2 addresses. ES-IS has an equivalent role as Address Resolution Protocol (ARP) in IP version 4 (IPv4).
ES-IS provides the basic interaction between Connectionless Network Service (CLNS) hosts (end systems) and routers (intermediate systems). ES-IS allows hosts to advertise NSAP addresses to other routers and hosts attached to the network. Those routers can then advertise the address to the rest of the network by using Intermediate System-to-Intermediate System (IS-IS). Routers use ES-IS to advertise their network entity title (NET) to hosts and routers that are attached to that network.
ES-IS routes are exported to Layer 1 IS-IS by default. You can also export ES-IS routes into Layer 2 IS-IS by configuring a routing policy. ES-IS generates and receives end system hello (ESH) hello messages when the protocol is configured on an interface. ES-IS is a resolution protocol that allows a network to be fully ISO integrated at both the network layer and the data layer.
The resolution of Layer 3 ISO NSAPs to Layer 2 subnetwork point of attachments (SNPAs) by ES-IS is equivalent to ARP within an IPv4 network. If a device is a provider edge (PE) router within a CLNS island that contains any end systems, you must configure ES-IS on the device.
For more information about ES-IS, see the ISO 9542 standard.