OSPF segment routing

IPv4 OSPF segment routing support is enabled through MPLS. OSPF creates an adjacency segment per OSPF neighbor, for a given interface, adjacency, and area. A separate MPLS label is allocated for each adjacency segment created. 

Labels are allocated only when the neighbor moves from the Init state to Upstate and requests the label manager for an unreserved label. The corresponding label transitions are downloaded to the MPLS forwarding table after the label is advertised in locally originated LSPs. In the case of LAN adjacencies, OSPF neighborship remains in a two-way state for the adjacencies between the DR-others. A separate label is allocated for each of the LAN neighbors, including the DR-other adjacencies that remain in the two-way state. 

The Junos OSPF implementation enables the network operator to provision the IPv4 address family node segment index node-sid. This node-sid will be assigned to a router and used by all other remote routers in the network to index into respective node segment label blocks (SRGBs). It derives the segment identifier to forward IPv4 traffic destined for the same router which was assigned as node-sid.