About SR-TE Policies
Segment routing for traffic engineering (SR-TE) uses a “policy” to steer traffic through the network. An SR-TE policy path is expressed as a list of segments that specifies the path, called a segment ID (SID) list. Each segment is an end-to-end path from the source to the destination, and instructs the routers in the network to follow the specified path instead of the shortest path calculated by the IGP. If a packet is steered into an SR-TE policy, the SID list is pushed on the packet by the head-end. The rest of the network executes the instructions embedded in the SID list.
There are two types of SR-TE policies: dynamic and explicit.
Local Dynamic SR-TE Policy
When you configure local dynamic SR-TE, the head-end locally calculates the path to the destination address. Dynamic path calculation results in a list of interface IP addresses that traffic engineering (TE) maps to adj-SID labels. Routes are learned by way of forwarding adjacencies over the TE tunnel.
Explicit SR-TE Policy
An explicit path is a list of IP addresses or labels, each representing a node or link in the explicit path. This feature is enabled through the explicit-path command that allows you to create an explicit path and enter a configuration submode for specifying the path.