BigIron RX Series Configuration Guide 85953-1001986-01Configuring IS-IS (IPv4) 28NOTESince the Brocade implementation of IS-IS does not route OSI traffic but instead routes IP traffic, IPhosts are shown instead of ESs.The other basic IS-IS concepts illustrated in this figure are explained in the following sections.Domain and areasIS-IS is an IGP, and thus applies only to routes within a single routing domain. However, you canconfigure multiple areas within a domain. A device can be a member of one area for each NetworkEntity Title (NET) you configure on the device. The NET contains the area ID for the area the NET isin.In Figure 117, Routers A, B, and C are in area 1. Routers D and E are in area 2. All the routers are inthe same domain.Level-1 routing and Level-2 routingYou can configure an IS-IS router such as a device to perform one or both of the following levels ofIS-IS routing1 :• Level-1 – A Level-1 router routes traffic only within the area the router is in. To forward traffic toanother area, the Level-1 router sends the traffic to its nearest Level-2 router.• Level-2 – A Level-2 router routes traffic between areas within a domain.In Figure 117 on page 858, Routers A and B are Level-1s only. Routers C and D are Level-1 andLevel-2 ISs. Router E is a Level-1 IS only.Neighbors and adjacenciesA device configured for IS-IS forms an adjacency with each of the IS-IS devices to which it is directlyconnected. An adjacency is a two-way direct link (a link without router hops) over which the twodevices can exchange IS-IS routes and other protocol-related information. The link is sometimescalled a “circuit”. The devices with which the device forms adjacencies are its neighbors, which areother ISs.A BigIron RX IS-IS interfaces are configured by default for broadcast circuits.In Figure 117 on page 858, Router A has an IS-IS adjacency with Router B. Likewise, Router B hasan IS-IS adjacency with Router A and Router C.Designated ISA Designated IS is an IS-IS router that is responsible for gathering and distributing link stateinformation to other Level-1 or Level-2 ISs within the same broadcast network (LAN). The Level-1and Level-2 Designated ISs within a broadcast network are independent, although the same devicecan be a Level-1 Designated IS and a Level-2 Designated IS at the same time.1. The ISO/IEC specifications use the spelling “routeing”, but this document uses the spelling“routing” to remain consistent with other Brocade documentation.
