Segment routing is a type of source routing, and in order to understand it, it is important to first understand source routing.
Source routing is a routing methodology that allows the sender to either partially or fully determine the route a packet will take through the network. This is in contrast to traditional routing where routing decisions are made incrementally at each router/node along the path. This allows for easier troubleshooting and allows a host to “know” all of the possible paths to the destination.
Segment routing is a type of source routing that is being developed by the IETF. In a segment routed network, an ingress router may prepend a header to packets that contain a list of segments, which are instructions that are executed on subsequent routers in the network. These instructions may be forwarding instructions, such as an instruction to forward a packet to a specific destination or interface.