the packet reaches a Level 1–2 or Level 2 IS in the destination area. Within the destination area, ISs forward the packet along the best path, based on system ID, until the packet reaches the destination. Note
Level 2 routing is also called interarea routing. Level 3 Routing
Routing between separate domains is called Level 3 routing. Level 3 routing is comparable to Border Gateway Protocol (BGP) interdomain routing in IP. Level 3 routing passes traffic between different autonomous systems, which might have different routing logic but might not have metrics that can be directly compared. Level 3 OSI routing is not implemented on Cisco routers.

Summary of Routing Levels
Routing levels can be summarized as follows:
Content 4.1 IS-IS Fundamentals 4.1.10 IS-IS and OSPF Network Design With OSPF, network design is constrained by the fact that OSPF is based on a central backbone (area 0), with all other areas being physically attached to area 0. The border between areas is inside the ABRs and each link is only in one area. When you use this type of hierarchical model, a consistent IP addressing structure is necessary to summarize addresses into the backbone. Summarization also reduces the amount of information carried in the backbone and advertised across the network. In comparison, IS-IS has a hierarchy of Level 1 and Level 2 or Level 1–2 routers, and the area borders lie on links. IS-IS permits a more flexible approach to extending the backbone by simply adding more Level 2 and Level 1–2 routers, which is a less complex process than with OSPF.
Figure summarizes these differences.
Content 4.1 IS-IS Fundamentals 4.1.11 Differences Between Integrated IS-IS and OSPF Although OSPF and IS-IS are very similar, there are some differences between the two routing protocols. OSPF runs on top of IP, whereas IS-IS runs through CLNS. OSPF produces many small LSAs. IS-IS updates are grouped by the router and sent as one LSP; therefore, the number of IS-IS updates is not an issue as network complexity increases. However, each update packet must be routed, and routing takes network resources, so more packets represent a larger impact on the network. Since IS-IS uses significantly fewer LSPs, at least 1,000 routers can reside in a single area, making IS-IS more scalable than OSPF. IS-IS is also more efficient than OSPF in the use of CPU resources and in the way it processes routing updates. Not only are there fewer LSPs to process (LSAs, in OSPF terminology) but also the mechanism by which IS-IS installs and withdraws prefixes is less intensive because it uses network entity title (NET) addresses, which are already summarized. The NET addresses used by IS-IS are covered later in this module. Both OSPF and IS-IS are link-state protocols and therefore provide fast convergence. The convergence time depends on a number of factors, such as timers, number of nodes, and type of router. Based on the default timers, IS-IS detects a failure faster than OSPF; therefore, convergence occurs more rapidly. If there are many neighboring routers and adjacencies, the convergence time may also depend on the processing power of the router. IS-IS is less CPU-intensive than OSPF. New ideas are not easily expressed in OSPF packets; they require the creation of a new LSA. The OSPF description schema is difficult to extend because of compatibility issues and due to being developed exclusively for IPv4. IS-IS is easy to extend through the Type, Length, Value (TLV) mechanism. TLV strings, called tuples, encode all IS-IS updates. IS-IS can easily grow to cover IPv6 or any other protocol, because extending IS-IS consists simply of creating new type codes. Similar to a designated router (DR) in OSPF, IS-IS elects a designated IS router (DIS). However, IS-IS does not elect a backup DR like OSPF does. This means that IS-IS repeats the election process whenever a new router becomes active. If the new router has a higher priority or the same priority and a higher system ID, it takes over as the DIS. Figure list some differences between the protocols relating to DR elections. A company may choose OSPF over IS-IS, because OSPF is more optimized and was designed exclusively as an IP routing protocol. For example, OSPF defines different area types (normal, stub, and not-so-stubby [NSSA]). The default OSPF metric is related to the interface bandwidth, while IS-IS defaults to a metric of 10 on all interfaces. If a company does choose OSPF, it requires networking equipment that supports OSPF and network engineers that are familiar with OSPF theory and operation. It is relatively easy to find both equipment and personnel to support an OSPF infrastructure. Furthermore, OSPF documentation is much more readily available than documentation for IS-IS. Figure summarizes the differences between OSPF and Integrated IS-IS. Interactive Media Activity Checkbox: OSPF versus IS-IS Upon completion of this activity, the student will be able to identify the differences between OSPF and IS-IS.
Content 4.2 ISO Addressing 4.2.1 NSAP Addresses Unlike IP addresses, CLNS addresses apply to entire nodes and not to interfaces. Because IS-IS was originally designed for CLNS, it requires CLNS addresses, even if the router is used for routing IP only. CLNS addresses that are used by routers are called network service access points (NSAPs). One part of an NSAP address is the NSAP selector (NSEL) byte. When a NSAP is specified with an NSEL of 0, it is called the NET. IS-IS link-state packets use NSAP addresses to identify the router and build the topology table and the underlying IS-IS routing tree; therefore, IS-IS requires NSAP addresses to function properly, even if they are used only for routing IP. NSAP addresses contain the following: The NSAP address is equivalent to the combination of the IP address and upper layer protocol number in an IP header. NSAP addresses have a maximum size of 20 bytes. The high-order bits identify the interarea structure, and the low-order bits identify unique systems within an area. There are a variety of NSAP address formats. Figure displays just three types of NSAP addresses. Although the fields preceding the System ID are different in each example, the System ID is the same.
Content 4.2 ISO Addressing 4.2.2 NSAP Address Structure The Cisco implementation of Integrated IS-IS divides the NSAP address into three fields: The variable-length area address is used for Level 2 routing, while the system ID and NSEL fields are used for Level 1 routing. Cisco routers routing CLNS use addressing that conforms to the ISO 10589 standard. ISO NSAP addresses consist of the following. IDP
The initial domain part (IDP) corresponds roughly to an IP classful major network. The IDP consists of an authority and format identifier (AFI) and an initial domain identifier (IDI):