IS-IS can announce node Entropy Label Capability (ELC), the Maximum Segment Depth (MSD) for node Entropy Readable Label Depth (ERLD) and the MSD for node Base MPLS Imposition (BMI). If needed, exporting the IS-IS extensions into BGP-LS requires no additional configuration. These extensions are standardized through draft-ietf-isis-mpls-elc-10, Signaling Entropy Label Capability and Entropy Readable Label Depth Using IS-IS, and RFC 8491, Signaling Maximum SID Depth (MSD) Using IS-IS.
When entropy and segment routing are enabled on a router, it automatically announces the ELC, ERLD, and BMI IS-IS values when IS-IS prefix attributes and router capabilities are announced. The following configuration logic is used.
The router automatically announces ELC for host prefixes associated with an IPv4 or IPv6 node SID when segment-routing , segment-routing entropy-label, and prefix-attributes-tlv are enabled for IS-IS. Although the ELC capability is a node property, it is assigned to prefixes to allow inter-area or inter-AS signaling. Consequently, the prefix-attribute TLV must be enabled accordingly within IS-IS.
The router announces the maximum node ERLD for IS-IS when segment-routing and segment-routing entropy-label are enabled together with advertise-router-capability.
The router announces the maximum node MSD-BMI for IS-IS when segment-routing and advertise-router-capability are enabled.
Exporting ELC, MSD-ERLD, and MSD-BMI IS-IS extensions into BGP-LS encoding is enabled automatically when database-export for BGP-LS is configured.
The announced value for maximum node MSD-ERLD and MSD-BMI can be modified to a smaller number using the override-bmi and override-erld commands. This can be useful when services (such as EVPN) or more complex link protocols (such as Q-in-Q) are deployed. Provisioning correct ERLD and BMI values helps controllers and local Constrained Shortest Path First (CSPF) to construct valid segment routing label stacks to be deployed in the network.
Segment routing parameters are configured in the following contexts:
configure>router>isis>segment-routing>maximum-sid-depth
configure>router>isis>segment-routing>maximum-sid-depth>override-bmi value
configure>router>isis>segment-routing>maximum-sid-depth>override-erld value