Intermediate System-to-Intermediate System Protocol

What Does Intermediate System-to-Intermediate System Protocol Mean?

Intermediate System-to-Intermediate System Protocol (IS-IS protocol) is an Interior Gateway Protocol that uses packet-switched networks to support efficient autonomous system routing for Internet service providers and large enterprises.

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IS-IS was originally defined by the International Organization for Standardization and International Electrotechnical Commission as ISO/IEC 10589:2002. The Internet Engineering Task Force (IETF) also published IS-IS as RFC 1142.

IS-IS is also known as integrated IS-IS.

Techopedia Explains Intermediate System-to-Intermediate System Protocol

IS-IS is based on the OSI model and assigns IS-IS nework router addresses to facilitate routing, bandwidth scalability and convergence. Routers build link state packets (LSP) based on local IS-IS interfaces and adjacent router prefixes. The routers flood LSPs to adjacent routers, and packets are encapsulated in the data link layer. IS-IS then adapts to Internet Protocol data transfer.

Key IS-IS features include:

Hierarchical routing
Fast convergence
Flexible timer tuning
Rapid LSP data flooding

Scalability

IS-IS routing components include a routing database that holds link state and forwarding databases, as well as the following four processes:

Receive: Includes data entry point (user/routing data, error reports, control packets), forwarding process user data, and error reports and update process routing data and control packets.

Update: Generates local LSPs flooded to adjacent routers and receives processes; forwards LSPs from adjacent routers.

Decision: Runs the open-shortest-path-first algorithm based on the LSP database and creates the forwarding database. Next hop information and equal cost path sets create load balancing adjacency sets.

Forward: Compiles received LSPs. Forwarding database transmits LSPs to destination points. Redirects load sharing and generates error reports.

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Margaret Rouse
Technology Expert

Margaret is an award-winning technical writer and teacher known for her ability to explain complex technical subjects to a non-technical business audience. Over the past twenty years, her IT definitions have been published by Que in an encyclopedia of technology terms and cited in articles by the New York Times, Time Magazine, USA Today, ZDNet, PC Magazine, and Discovery Magazine. She joined Techopedia in 2011. Margaret's idea of a fun day is helping IT and business professionals learn to speak each other’s highly specialized languages.