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22Intermediate System to Intermediate SystemIntermediate system to intermediate system (Is-IS) is supported on Dell Networking OS.• IS-IS is supported on the Z9000 with Dell Networking OS 9.0(0.0).•• The IS-IS protocol is an interior gateway protocol (IGP) that uses a shortest-path-first algorithm. Dell Networking supports bothIPv4 and IPv6 versions of IS-IS.• The IS-IS protocol standards are listed in the Standards Compliance chapter.IS-IS Protocol OverviewThe IS-IS protocol, developed by the International Organization for Standardization (ISO), is an interior gateway protocol (IGP) thatuses a shortest-path-first algorithm.NOTE: This protocol supports routers passing both IP and OSI traffic, though the Dell Networking implementationsupports only IP traffic.IS-IS is organized hierarchically into routing domains and each router or system resides in at least one area. In IS-IS, routers aredesignated as Level 1, Level 2 or Level 1-2 systems. Level 1 routers only route traffic within an area, while Level 2 routers route trafficbetween areas. At its most basic, Level 1 systems route traffic within the area and any traffic destined for outside the area is sent toa Level 1-2 system. Level 2 systems manage destination paths for external routers. Only Level 2 routers can exchange data packetsor routing information directly with external routers located outside of the routing domains. Level 1-2 systems manage both inter-areaand intra-area traffic by maintaining two separate link databases; one for Level 1 routes and one for Level 2 routes. A Level 1-2 routerdoes not advertise Level 2 routes to a Level 1 router.To establish adjacencies, each IS-IS router sends different protocol data units (PDU). For IP traffic, the IP addressing information isincluded in the IS-IS hello PDUs and the link state PDUs (LSPs).This brief overview is not intended to provide a complete understanding of IS-IS; for that, consult the documents listed in Multi-Topology IS-IS.IS-IS AddressingIS-IS PDUs require ISO-style addressing called network entity title (NET).For those familiar with name-to-network service mapping point (NSAP) addresses, the composition of the NET is identical to anNSAP address, except the last byte is always 0. The NET is composed of the IS-IS area address, system ID, and N-selector. The lastbyte is the N-selector. All routers within an area have the same area portion. Level 1 routers route based on the system addressportion of the address, while the Level 2 routers route based on the area address.The NET length is variable, with a maximum of 20 bytes and a minimum of 8 bytes. It is composed of the following:• area address — within your routing domain or area, each area must have a unique area value. The first byte is called theauthority and format indicator (AFI).• system address — the router’s MAC address.• N-selector — this is always 0.The following illustration is an example of the ISO-style address to show the address format IS-IS uses. In this example, the first fivebytes (47.0005.0001) are the area address. The system portion is 000c.000a.4321 and the last byte is always 0.352 Intermediate System to Intermediate System