Route calculation and selection

About this task

The Designated IS uses a Shortest Path First (SPF) algorithm to calculate paths to destination ISs and ESs. The SPF algorithm uses Link State PDUs (LSPDUs) received from other ISs as input, and creates the paths as output.

After calculating the paths, the Designated IS then selects the best paths and places them in the IS-IS route table. The Designated IS uses the following process to select the best paths.

Procedure

  1. Prefer the Level 1 path over the Level 2 path.
  2. If there is no Level 1 path, prefer the internal Level 2 path over the external Level 2 path.
  3. If there is still more than one path, prefer the path with the lowest metric.
  4. If there is more than one path with the lowest metric, load share among the paths.

    After selecting the best path to a destination, the software places the path in the IS-IS route table.

9036959-00 Rev AA