Intermediate System-to-Intermediate System (IS-IS)

The International Organization for Standardization (ISO) came up with a routing protocol to use with the Open System Interconnection (OSI) protocol suite. The ISO protocol suite includes the Intermediate System-to-Intermediate System (IS-IS) protocol, End System-to-Intermediate System (ES-IS) protocol, and Interdomain Routing Protocol (IDRP).

The IS-IS protocol was originally used in ISO Connectionless Network Protocol (CLNP) networks. A version of IS-IS has been created to work on both the CLNP and IP networks. This version is referred to as integrated IS - IS (also referred to as dual IS - IS ). You can find more information on the OSI protocol suite in the ISO document that defines IS-IS, which is ISO 10589.

The ISO networking model uses some specific terminology. These terms are the basis for the ES-IS and IS-IS OSI products. The ES-IS protocol enables ES and IS to discover each other. The IS-IS protocol provides routing between ISs. The following are some important terms relating to IS-IS:

• Area ” A group of contiguous networks and an attached host specified to be an area by the network administrator

• Domain ” A collection of connected areas; for example, a routing domain provides full connectivity to all end systems within the domain

• End system (ES) ” Any non-routing network nodes

• Intermediate system (IS) ” A router

• Level 1 routing ” Routing within a level 1 area

• Level 2 routing ” Routing between two level 1 areas

IS-IS is an OSI link-state hierarchical routing protocol that floods the network with link-state information in order to build a complete picture of the network topology. To simplify the router design and operation, IS-IS distinguishes between two levels of IS. Level 1 IS communicates within a level 1 area, and level 2 IS routes between level 1 areas and creates an interdomain routing backbone. IS-IS uses a default metric with a maximum path value of 1,024. The use of the metric is arbitrary and usually assigned by the network administrator. Any single link can have a maximum value of 64, and path links are calculated by summing the link values.

IS-IS also defines three optional metrics (cost):

• Delay ” Deals with the delay on the link

• Expense ” Deals with the communication cost associated with the link

• Error ” Deals with the error rate on the link

IS-IS maintains a mapping of these metrics to the Quality of Service (QoS) option in the CLNP packet header. IS-IS uses the mapping to compute routes through the internetwork.

IS-IS uses three basic packet formats: hello packets, link-state packets, and sequence-numbers packets (SNPs). Each of the packet formats has a complex format with three different logical parts . The first is an 8-byte fixed header shared by all three packet formats. The second is packet type specific (either hello, link state, or sequence-numbers packets) with a fixed format. The third is also packet-type specific (either hello, link state, or sequence- numbers packets), but it is a variable length.