The BGP-LS extension allows IGPs (OSPF/IS-IS) link state database information to be injected into BGP. This is typically used in deployments where some external component, (like a controller or Path Computation Engine) can do centralized path computations by learning the entire IGP topology through BGP-LS. The controller can then communicate the computed paths based on the BGP-LS updates to the head end device in the network. The mechanism used by the controller to communicate the computed TE paths is outside the scope of this document. Using BGP-LS instead of an IGP peering with the controller to distribute IGP link state information has the following advantages.

Segment Routing Traffic Engineering Policy (SR TE) aka SR Policy makes use of Segment Routing (SR) to allow a headend

By default if there's a hostname configured on the switch, it is used as the IS IS hostname. It is also possible to