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

Techopedia explains Intermediate System-to-Intermediate System Protocol (IS-IS 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.