burden on routers supporting the protocol. IS-IS routing takes place at Level 1 and Level 2 within an autonomous system. Level 1 routing occurs within an IS-IS area. It recognizes the location of ISs and end systems (ESs), such as computers and hosts. It then builds a routing table to reach each system. All devices in a Level 1 routing area have the same area address. Routing within an area is accomplished by looking at the locally significant address portion (known as the system ID) and choosing the lowest cost path. Level 2 routers learn the locations of Level 1 routing areas and build an interarea routing table. All ISs in a Level 2 routing area use the destination area address to route traffic using the lowest cost path. Figure summarizes key IS-IS terms. Interactive Media Activity Checkbox: IS-IS Terminology Quiz Upon completion of this activity, the student will know the IS-IS terminology. Web Links Intermediate System-to-Intermediate System Protocol
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Content 4.1 IS-IS Fundamentals 4.1.5 IS-IS Link-State Operation To support the two 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. Note
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.
Content 4.1 IS-IS Fundamentals 4.1.6 Integrated IS-IS Integrated IS-IS or dual IS-IS is an implementation of the IS-IS protocol for routing two different network protocols in the same network concurrently. The two network protocols are IP and CLNS (specified in RFC 1195 and ISO 10589). Integrated IS-IS tags CLNP routes with information about IP networks and subnets. Integrated IS-IS provides IP with an alternative to OSPF and combines ISO CLNS and IP routing in one protocol. Integrated IS-IS can be used for IP routing, CLNS routing, or a combination of the two. Integrated IS-IS uses its own protocol data units (PDUs) to transport information between routers, including IP reachability information. IS-IS information is not carried within a network-layer protocol; instead, it is carried directly within data link layer frames. Note
This protocol independence makes IS-IS easily extensible. There is also a version of Integrated IS-IS that supports IP version 6 (IPv6). Since IS-IS uses CLNS addresses to identify the routers and to build the LSDB, an understanding of CLNS addresses is required to configure and troubleshoot IS-IS, even when it is used only for routing IP.
Content 4.1 IS-IS Fundamentals 4.1.7 Principles and Issues of IS-IS Design Effective networks are well planned. The first and most important step in building a scalable network is developing a good addressing plan that allows for route summarization. Route summarization is possible only when using a hierarchical addressing structure. Effective address planning presents opportunities to group devices into areas. Using areas confines the scope of LSP propagation and saves bandwidth. The logical place to implement route summarization is on the Level 1–2 routers, which is on the border between a Level 1 area and the Level 2 backbone. Route summarization saves memory because each IS is no longer responsible for the LSPs of the entire routing domain. Route summarization also saves CPU usage because a smaller routing table is easier to maintain and route lookups are quicker. One issue with IS-IS is that older implementations using the narrow metrics are limited to a maximum interface metric of 63 (6 bits) and a maximum total path metric of 1,023 (10 bits). As a result, there is little room to distinguish between paths. Cisco IOS software, beginning with Software Release 12.0, supports wide metrics that allow a 24-bit interface and 32-bit path metrics. The default, however, is still the narrow metrics. Note
Complications can occur if you use wide metrics along with narrow metrics (for example, on older routers or in a multivendor environment). IS-IS as implemented on Cisco routers does not automatically scale the interface metric. Instead, all IS-IS interfaces have a default metric of 10. This setting can be changed manually. If the default metric is not adjusted on each interface, the IS-IS metric becomes similar to the hop-count metric used by the Routing Information Protocol (RIP).
Content 4.1 IS-IS Fundamentals 4.1.8 The ES-IS Protocol Hosts in the OSI terminology are called end systems (ESs). The End System-to-Intermediate System (ES-IS) protocol permits ESs (hosts) and ISs (routers) to discover one another. ES-IS handles topology information discovery and exchange between ESs and ISs. ES-IS also allows ESs to learn their network-layer addresses. ES-IS performs the following tasks: ESs send End System Hellos (ESHs) to well-known addresses that announce their presence to ISs. Routers listen to ESHs to find the ESs on a segment. Routers include information about ESs in LSPs. Routers transmit Intermediate System Hellos (ISHs) to well-known addresses, announcing their presence to ESs. ESs listen for these ISHs and randomly pick an IS to which they forward all their packets. When an ES needs to send a packet to another ES, it sends the packet to one of the ISs (routers) on its directly attached network.
Routers use IS-IS Hellos (IIHs) for establishing and maintaining adjacencies between ISs. IP systems do not use ES-IS. IP has its own processes and applications to handle the same functions as ES-IS, such as Internet Control Message Protocol (ICMP), Address Resolution Protocol (ARP), and DHCP. Although Integrated IS-IS can support IP exclusively, IS-IS still uses CLNS to transmit reachability information and still forms adjacencies using IIHs.
Content 4.1 IS-IS Fundamentals 4.1.9 OSI Routing Levels The OSI specifications discuss four unique types of routing operations, which are numbered from 0 to 3. As discussed earlier, IS-IS is responsible for Level 1 and Level 2 OSI routing. Level 0 Routing
OSI routing begins with ES-IS when the ESs discover the nearest IS by listening to ISH packets. When an ES needs to send a packet to another ES, it sends the packet to an IS on an attached network. This process is known as Level 0 routing.

IS-IS Level 1 Routing
Each ES and IS resides in a particular area. To pass traffic, the router looks up the destination address and forwards the packet using the best route. If the destination is on the same subnetwork, the IS is aware of the location (from listening to the ESH) and forwards the packet appropriately. The IS can also provide a redirect message to the source that tells it that a more direct route is available. If the destination is on a different subnetwork but within the same area, the router identifies the best path using the system ID and forwards the traffic appropriately. Note
Level 1 routing is also called intra-area routing. IS-IS Level 2 Routing
If a destination address is in another area, the Level 1 IS sends the packet to the nearest Level 1–2 IS. This process is called Level 2 routing. Packet forwarding continues through Level 2 ISs until