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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.


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


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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