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Configuring Segment Routing Global Blocks Label Ranges in SPRING for IS-IS Protocol

Segment routing (SR) or source packet routing in networking (SPRING) is a control-plane architecture that enables an ingress router to steer a packet through a specific set of nodes and links without relying on the intermediate nodes in the network to determine the actual path it should take. The label range for a segment routing global block (SRGB) is the range of label values used in segment routing. You can configure the start of the label range and the index range. The end of the label range is the summation of the start label value and the index range.

Before you configure SPRING SRGB for ISIS protocol, you must:

  • Configure the router interfaces.

  • Configure ISIS.

To configure SPRING SRGB label range on a device:

  1. Configure the start-label and index-range of SRGB. The start label value indicates the start of the SPRING label block and the index range along with the start label indicate the end of the label block.
    • Ensure that the MPLS label for a binding segment ID (SID) is the sum of the SRGB start label and SID index value. In addition, SID index value must be less than or equal to the index-range value specified in the configuration.

    • Junos does not check whether the SID index is within the SRGB's range when the SID index is assigned through an ISIS export policy. If you configure an index that is out of range of the configured SRGB, you won’t see any error message in the logs or while committing the configuration. Junos OS shows a commit error only when you configure the SID under the [edit protocols isis source-packet-routing] hierarchy level.


    The default value for the index range is 4096. This causes chunks of 256 label blocks being dynamically allocated by the label manager depending on the availability.

    For example, configure SRGB with start-label 800,000 and index-range 40,000. The start label of the SPRING label block is 800,000 and the end of the label block is 840,000.


    Ensure that the labels in the SRGB label range are not used by any other applications. If a label in the configured label range is used by another application, then a syslog error message RPD_ISIS_SRGBALLOCATIONFAIL is logged to indicate that the label manager is unable to allocate the requested SRGB label range. To free up the configured label range, check the label ranges configured at the [edit protocol mpls label-range] hierarchy level and re-configure the SRGB label range with a label range that is available and restart the routing protocol process (RPD).

  2. Configure the value of IPv4 node segment index.

    For example, configure 1001 for IPv4 node segment index.

  3. Configure the value of IPv6 node segment index.

    For example, configure 2001 for IPv6 node segment index.