Segment Routing (SR) allows a head end node to steer a packet flow along any path. Intermediate per-flow states are eliminated due to source routing. The head end node is said to steer a flow into a Segment Routing Policy (SR Policy). The header of a packet steered in an SR Policy is augmented with the ordered list of segments associated with that SR Policy. This feature is supported for MPLS instantiations.

BGP-LS describes a mechanism by which Link-State (LS) and Traffic Engineering (TE) information from IGPs can be collected from networks and shared with external components using the BGP. This is achieved using a new BGP Network Layer Reachability Information (NLRI) encoding format. The mechanism is applicable to physical and virtual links. Applications of this technique include Path Computation Elements (PCEs). These components, while external to the network, require network state information on a real time basis. Specifically, they require link-state database information of each IGP node from the entire network. BGP protocol is used to collect the necessary information and to share with the external components and this is achieved using a NLRI encoding format.

An NLRI is defined to advertise SR Policy to the headend of that policy. New sub-TLVs for the Tunnel Encapsulation Attribute are defined to carry SR related information.