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Configuring Flexible Algorithm for Segment Routing Traffic Engineering

Before you begin configuring the flexible algorithm for IS-IS, make sure you:

  1. Configure the device interfaces to enable IP transport.

  2. Configure IS-IS protocol to enable dynamic routing protocol to exchange routing information.

  3. Configure BGP protocol.

  4. Configure segment routing.

To configure flexible algorithm for IS-IS:

  1. Define flexible algorithm on routers that you have identified in your network. Assign an ID for the flexible algorithm definition (FAD) ranging from 128 through 255.

    We recommend configuring flexible algorithm on only a few routers to provide redundancy and to avoid conflicts.

    Specify the parameters of the definition. IS-IS calculates the path based on these specified parameters of the FAD.

    1. Map a BGP color community to the defined FAD. By default each flexible algorithm is associated with a value equal to the flex algorithm.

      VPN can be made to resolve paths over the configured BGP color community.


      Changing the BGP color community for a flexible algorithm might result in traffic disruption. If you modify a BGP color community for a flexible algorithm then all routes pertaining to that flexible algorithm are removed from the RIB and added again with new colors.

    2. Specify the calculation type based on which the IS-IS protocol calculates the path.
    3. Specify the metric type based on which IS-IS calculates the path.
    4. Assign a priority level to the advertisement of the FAD based on your requirement. Specify a priority ranging from 0 through 255.

      Modifying the flexible algorithm definition could cause traffic disruptions until all the nodes converge on the new paths.

    5. If you have enabled RSVP traffic engineering, you can configure admin-groups for many protocols to color an individual link.
    6. Define the admin groups as per your requirement.

      For FADs with link-constraints to work, all relevant links should advertise the admin-colors in IS-IS. You must either enable RSVP on the interfaces or if you have not configured RSVP for traffic engineering, make sure you configure set traffic-engineering advertise always at the [edit protocols isis] hierarchy level.

  2. Identify the participating routers and configure participation on those routers. The same device can advertise a FAD and also participate in a flexible algorithm.
  3. Advertise prefix segments through policy configuration.
  4. Apply the policy under the protocol IS-IS.
  5. To verify if your flexible algorithm configuration is working correctly use the show isis spring flex-algorithm command.