Some labels that are provisioned through CLI or a management interface must be allocated from the Segment Routing Local Block (SRLB). The SRLB references a reserved label block configured under config>router>mpls-labels. Refer to the 7450 ESS, 7750 SR, 7950 XRS, and VSR MPLS Guide for further information on reserved label blocks.
The label block to use is specified by the srlb command under IS-IS or OSPF:
config>router>isis>segment-routing
[no] srlb <reserved-label-block-name>
config>router>ospf> segment-routing
[no] srlb <reserved-label-block-name>
Provisioned labels for adjacency SIDs and adjacency SID sets must be allocated from the configured SRLB. If no SRLB is specified, or the requested label does not fall within the SRLB, or the label is already allocated, then the request is rejected.