DSP
The domain specific part (DSP) contributes to routing within an IS-IS routing domain. The DSP comprises the high-order DSP (HO-DSP), the system ID, and the NSEL.
Content 4.2 ISO Addressing 4.2.3 NSAP Address Example The simplest NSAP format, which is used by most companies running IS-IS as their IGP, includes the following: Note
The NSAP is called the NET when it has an NSEL of 0. Routers use the NET to identify themselves in the IS-IS protocol data units. For example, you might assign 49.0001.0000.0c12.3456.00, which represents the following: Note
The area address is also referred to as the prefix.
Note
Some IS-IS documentation uses the terms “area ID” and “area address” as synonyms. Regardless of the format, the following three rules apply:
Content 4.2 ISO Addressing 4.2.4 Identifying Systems in IS-IS The first part of an NSAP is the area address and it is associated with the IS-IS routing process. Unlike OSPF, an IS-IS router can be a member of only one area. All routers in an area must use the same area address, which actually defines the area. The area address is used in Level 2 routing. End systems (ESs) recognize only intermediate systems (ISs) and other ESs on the same subnetwork that share the same area address. The 6-byte NSAP system ID must be unique within an area. It is customary to use a MAC address from the router or, for Integrated IS-IS, to encode an IP address into the system ID. All the system IDs in a domain must be of equal length. Cisco enforces this OSI directive by fixing the length of the system ID at 6 bytes in all cases. Level 1 intra-area routing is based on system IDs; therefore, each ES and IS must have a unique system ID within the area. All Level 2 ISs eventually recognize all other ISs in the Level 2 backbone, so they must also have unique system IDs. System IDs must remain unique across the domain to avoid conflict at Level 1 or Level 2 if, for example, a device moves into a different area.
Content 4.2 ISO Addressing 4.2.5 NET Addresses As discussed earlier, NSAP addresses have a one-octet NSEL field that identifies a process on the device, which corresponds roughly to a port number in IP. NET addresses are NSAP addresses with an NSEL value of 0. A NET address uniquely identifies an OSI host within an IS-IS routing domain. Because IS-IS originates from the OSI world, NET addresses are required even if the only routed protocol is IP. The NET refers to the device itself; that is, it is the equivalent of the Layer 3 OSI address of that device. Routers use the NET to identify themselves in the LSPs and to form the basis for the OSI routing calculation.
Content 4.2 ISO Addressing 4.2.6 Subnetwork Point of Attachment, Circuits, and Links The subnetwork point of attachment (SNPA) is the point that provides subnetwork services. The SNPA is the equivalent of the Layer 2 address corresponding to the NET or NSAP address. The SNPA is assigned by using one of the following: A circuit is the IS-IS term for an interface. Since the NSAP and NET refer to the entire device, a circuit ID is used to distinguish a particular interface. The router assigns a circuit ID (one octet) to each interface on the router as follows: A link is the path between two neighbor ISs and is defined as being up when communication is possible between the two neighbor SNPAs.
Content 4.2 ISO Addressing 4.2.7 IS-IS Routing Levels IS-IS defines three types of routers: The path of connected Level 2 and Level 1–2 routers is called the backbone. All areas and the backbone must be contiguous. Area boundaries fall on the links. Each IS-IS router belongs to exactly one area. Neighboring routers learn whether they are in the same area or different areas and negotiate appropriate adjacencies: Level 1, Level 2, or both. Each router keeps a copy of the LSDBs for the levels that it is responsible for. A Level 1–2 router automatically advertises to all Level 1 routers (within its area) that it is a potential exit point of the area. Level 1 routers default to the nearest attached Level 1–2 router. Interactive Media Activity Drag and Drop: Identifying IS-IS Routers Upon completion of this activity, the student will be able to identify the different types of IS-IS routers in a network.
Content 4.2 ISO Addressing 4.2.8 Intra-Area and Interarea Addressing and Routing Logic IS-IS routing flows naturally from the OSI address plan in which areas are identified and unique system IDs are given to each device.The area address portion of the NSAP address can range from 1 to 13 bytes in length, as specified by the ISO standard. Therefore, an NSAP for an IS-IS network can be as little as 8 bytes in length, but is usually longer to permit some granularity in the allocation of areas. The area address prefix is common to all devices in an area and unique for each area. ISs and ESs are in the same area if they share the same area address. Routing within an area involves collecting system IDs and adjacencies for all ISs and ESs in an area and using Dijkstra’s algorithm to compute best paths between devices. Level 1 routers are aware of the local area topology only. They pass the traffic destined outside their area to the closest Level 1–2 router. Routing between areas is based on the area address. Level 2 routers in different areas exchange area address information and use Dijkstra’s algorithm to compute best paths between areas. They pass traffic between areas to the closest Level 1–2 router. When an ES is required to send a packet to another ES, the packet goes to one of the ISs on a