Segment Routing With Traffic Engineering (SR-TE)

When segment routing is used together with MPLS data plane, the SID is a standard MPLS label. A router forwarding a packet using segment routing therefore pushes one or more MPLS labels.

Segment routing using MPLS labels can be used in both shortest path routing applications (see Segment Routing in Shortest Path Forwarding for more information) and in traffic engineering (TE) applications, as described in this section.

An SR-TE LSP supports a primary path, with Fast Reroute (FRR) backup, and one or more secondary paths. A secondary path can be configured as standby.

SR OS implements the following computation methods for the paths of a SR-TE LSP:

Hop-to-Label Translation — the TE-DB converts the list of hops, destination of the LSP and the strict or loose hops in the path definition, to a list of SIDs by searching the IGP instances with segment routing enabled. This method does not support TE constraints except for loose or strict hops.
See SR-TE LSP Path Computation Using Hop-to-Label Translation for more details.
Local CSPF — the LSP path TE constraints are considered in the path computation. This method implements most of the CSPF capabilities supported with RSVP-TE LSP with very few exception such as the bandwidth constraint which cannot be booked with SR-TE LSP because of the lack of a signaling protocol to establish the LSP path.
See SR-TE LSP Path Computation Using Local CSPF for more details.
Path Computation Element (PCE) — in this case the router acting as a PCE Client (PCC) requests a computation of the path of a SR-TE LSP from the PCE using the PCEP Protocol.
See Path Computation Element Protocol (PCEP) for more details.
User specified SID list — the SR-LSP feature provides the option for the user to manually configure each path of the LSP using an explicit list of SID values.
See SR-TE LSP Paths using Explicit SIDs for more details.

The configured or computed path of a SR-TE LSP can use a combination of node SIDs and adjacency SIDs.