3. Link-State IGP Segments

Within an SR domain, an SR-capable IGP node advertises segments for its attached prefixes and adjacencies. These segments are called "IGP segments" or "IGP SIDs". They play a key role in Segment Routing and use cases as they enable the expression of any path throughout the SR domain. Such a path is either expressed as a single IGP segment or a list of multiple IGP segments.

Advertisement of IGP segments requires extensions in link-state IGP protocols. These extensions are defined in [ISIS-SR-EXT], [OSPF-SR-EXT], and [OSPFv3-SR-EXT].