858 BigIron RX Series Configuration Guide53-1001986-01Configuring IS-IS (IPv4)28• If the path provided by IS-IS has the lowest administrative distance, then the CPU places thatIS-IS path in the IP route table.• If a path to the same destination supplied by another protocol has a lower administrativedistance, the CPU installs the other protocol’s path in the IP route table instead.The administrative distance is a protocol-independent value from 1 – 255. Each path sent to theCPU, regardless of the source of the path (IS-IS, OSPF, static IP route, and so on) has anadministrative distance.Each route source has a default administrative distance. The default administrative distance forIS-IS is 115.You can change the administrative distance for IS-IS and other routes sources.Intermediate systems and end systemsIS-IS uses the following categories to describe devices within an IS-IS routing domain (similar to anOSPF Autonomous System):• Intermediate System (IS) – A device capable of forwarding packets from one device to anotherwithin the domain. In Internet Protocol (IP) terminology, an IS is a router.• End System (ES) – A device capable of generating or receiving packets within the domain. In IPterminology, an ES is an end node or IP host.When you configure IS-IS on a device, the device is an IS.Figure 117 shows an example of an IS-IS network.FIGURE 117 An IS-IS network contains Intermediate Systems (ISs) and host systemsRouter A Router B Router C Router DRouter EBGP4IS-IS Area 1 IS-IS Area 2IS-IS Routing DomainIPHostIPHostIPHostAn IS-IS routing domain cancontain multiple areas.IS-IS routers route within anarea at Level-1.IS-IS routers route betweenareas at Level-2.
