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IS-IS is an International Organization for Standardization (ISO) dynamic routing specification. IS-IS is described in ISO/IEC 10589, reprinted by the Internet Engineering Task Force (IETF) as RFC 1142. IS-IS is a link-state routing protocol that floods link-state information throughout the network to build a picture of network topology. IS-IS was primarily intended to route OSI Connectionless Network Protocol (CLNP) packets but can also route IP packets. IP packet routing uses Integrated IS-IS, which provides the ability to route protocols such as IP. IS-IS is a common alternative to other powerful routing protocols such as OSPF and EIGRP in large networks. Although it isn't seen much in enterprise networks, IS-IS is commonly used for internal routing in large ISP networks.

IS-IS creates two levels of hierarchy, with Level 1 for intra-area and Level 2 for interarea routing. IS-IS distinguishes between Level 1 and Level 2 intermediate systems (IS). Level 1 ISs communicate with other Level 1 ISs in the same area. Level 2 ISs (routers) are configured for L1/L2 areas, which route between Level 1 areas and form an intra-domain routing backbone. Hierarchical routing simplifies backbone design because Level 1 ISs only need to know how to get to the nearest Level 2 IS.

Note

In IS-IS, a router is usually the IS, and PCs, workstations, and servers are end systems (ES). End System-to-Intermediate System links are Level 0.

IS-IS Metrics

IS-IS as originally defined uses a composite metric with a maximum path value of 1023. The required default metric is arbitrary and typically is assigned by a network administrator. By convention, it is intended to measure the circuit's capacity to handle traffic, such as its throughput in bits per second. Higher values indicate a lower capacity. Any single link can have a maximum value of 63. IS-IS calculates path values by summing link values. The standard sets the maximum metric values to provide the granularity to support various link types. It also ensures that the shortest-path algorithm used for route computation is reasonably efficient.

In Cisco routers, all interfaces have a default metric of 10. The administrator must configure the interface metric to get a different value. This small metric value range has proven insufficient for large networks. It also provides too little granularity for new features such as traffic engineering and other applications, especially with high-bandwidth links. Cisco IOS Software addresses this issue with the support of a 24-bit metric field, the so-called "wide metric." Wide metrics are also required for route leaking. Using the new metric style, link metrics now have a maximum value of 16,777,215 (2²⁴ – 1), with a total path metric of 4,261,412,864 (254 * 2²⁴ or 2³²). Deploying IS-IS in the IP network with wide metrics is recommended for enabling finer granularity and supporting future applications such as traffic engineering.

IS-IS also defines three optional metrics (costs): delay, expense, and error. Cisco routers do not support the three optional metrics. The wide metric noted earlier uses the octets reserved for these metrics.

IS-IS Operation and Design

This subsection discusses IS-IS areas, designated routers, authentication, and the NET. IS-IS defines areas differently from OSPF; area boundaries are links and not routers. IS-IS has no BDRs. Because IS-IS is an OSI protocol, it uses a NET to identify each router.

NET

To configure the IS-IS routing protocol, you must configure a NET on every router. Although configuring NET is not a CCDA test requirement, this information is included for "extra credit."

Although you can configure IS-IS to route IP, the communication between routers uses OSI PDUs. The NET is the OSI address used for each router to communicate with OSI PDUs. A NET address ranges from 8 to 20 bytes. It consists of a domain, area ID, system ID, and selector (SEL), as shown in Figure 11-8.

IS-IS routers use the area ID. The system ID must be the same length for all routers in an area. For Cisco routers, it must be 6 bytes in length. Usually, a router MAC address identifies each unique router. The SEL is configured as 00. You configure the NET with the net subcommand under the router isis command. In the following example, the domain authority and format identifier (AFI) is 49, the area is 0001, the system ID is 00aa.0101.0001, and the SEL is 00:

router isis
 net 49.0001.00aa.0101.0001.00

IS-IS DRs

As with OSPF, IS-IS selects DRs on multiaccess networks. It does not choose a backup DR as does OSPF. By default, the priority value is 64. You can change the priority value to a value from 0 to 127. If you set the priority to 0, the router is not eligible to become a DR for that network. IS-IS uses the highest system ID to select the DR if there is a tie with the priorities. On point-to-point networks, the priority is 0 because no DR is elected. In IS-IS, all routers in a multiaccess network establish adjacencies with all others in the subnetwork, and IS-IS neighbors become adjacent upon the discovery of one another. Both these characteristics are different from OSPF behavior.

IS-IS Areas

IS-IS uses a two-level hierarchy similar to the OSPF area hierarchy developed later. Routers are configured to route Level 1 (L1), Level 2 (L2), or both Level 1 and Level 2 (L1/L2). Level 1 routers are like OSPF internal routers in a Cisco totally stubby area. An L2 router is similar to an OSPF backbone router. A router that has both Level 1 and Level 2 routes is similar to an OSPF ABR. IS-IS does not define a backbone area, but you can consider the backbone a continuous path of adjacencies among Level 2 ISs.

The L1/L2 routers maintain a separate link-state database for the L1 routes and L2 routes. Also, the L1/L2 routers do not advertise L2 routes to the L1 area. L1 routers do not have information about destinations outside the area and use L1 routes to their L1/L2 router to reach outside destinations.

As shown in Figure 11-9, IS-IS areas are bounded not by the L1/L2 routers but by the links between L1/L2 routers and L2 backbone routers.

IS-IS Authentication

IS-IS supports three types of clear-text authentication: link authentication, area authentication, and domain authentication. All these types support only clear-text password authentication. Recently, an RFC draft added support for an IS-IS MD5.

Routers in a common subnetwork (Ethernet, private line) use link authentication. The clear-text password must be common only between the routers in the link. Level 1 and Level 2 routes use separate passwords.

With area authentication, all routers in the area must use the same authentication mode and must have the same password.

Only L2 and L1/L2 routers use domain authentication. All L2 and L1/L2 routers must be configured for the same authentication mode and must use the same password.

IS-IS for IPv6

The specification for routing IPv6 with integrated IS-IS is currently an Internet draft (draft-ietf-isis-ipv6-06.txt) of the IETF. The draft specifies new type, length, and value (TLV) objects, reachability TLVs, and an interface address TLV to forward IPv6 information in the network. IOS currently supports IS-IS for IPv6, as described in the draft standard. Because IS-IS for IPv6 is not a focus area for the CCDA, refer to the IETF drafts for further information.

IS-IS Summary

The characteristics of IS-IS follow:

• Link-state protocol.
• Uses OSI CLNP to communicate with routers.
• Classless protocol (supports VLSMs and CIDR).
• Default metric is set to 10 for all interfaces.
• Single metric: single link max = 63, path max = 1023.
• Sends partial route updates only when changes occur.
• Authentication with clear-text passwords and MD5.
• Administrative distance is 115.
• Used in large service provider networks. Not recommended for enterprise networks. Sometimes attractive compared to OSPF and EIGRP.
• Described in ISO/IEC 10589; reprinted by the IETF as RFC 1142.
• IETF draft for routing IPv6 with IS-IS.