CLNS Overview

CLNS uses network service access points (NSAPs), similar to IP addresses found in IPv4, to identify end systems (hosts) and intermediate systems (routers). ES-IS enables the hosts and routers to discover each other. IS-IS is the interior gateway protocol (IGP) that carries ISO CLNS routes through a network.

Depending on your network topology, one or more of the following components are needed to route within a CLNS environment:

• ES-IS—Provides the basic interaction between CLNS hosts (end systems) and routers (intermediate systems). Using ES-IS, hosts advertise their ISO NSAP addresses and subnetwork point-of-attachment (SNPA) addresses to other routers and hosts attached to the subnetwork. The resolution of Layer 3 ISO NSAPs to Layer 2 SNPAs by ES-IS is equivalent to ARP within an IPv4 network.

  If a CLNS island does not contain any end systems, you do not need to configure ES-IS on a device.

• IS-IS extensions—Provide the basic IGP support for collecting intradomain routing information for CLNS destinations within a CLNS network. Routers learning host addresses through ES-IS can advertise them to other routers (intermediate systems) using IS-IS.
• Static routes—You can configure static routes to exchange CLNS routes within a CLNS island. You can use static routing with or without IS-IS.
• Border Gateway Protocol (BGP) extensions—BGP extensions allow BGP to carry CLNS VPN network layer reachability information (NLRI) between PE routers. Each CLNS route is encapsulated into a CLNS VPN NLRI and propagated between remote sites in a VPN.

For more information about CLNS, see the ISO 8473 standards. For more information about IS-IS, see the ISO 10589 standard. For more information about ES-IS, see the ISO 9542 standard.