The head-end validates the path of an SR-TE LSP for connectivity against the TE topology. MPLS-TE head-end checks if links corresponding to the adjacency SIDs are connected in the TE topology.
For newly-instantiated SR-TE LSPs, if the head-end detects a discontinuity on any link of the SR-TE path, that path is considered invalid and is not used. If the tunnel has other path-options with valid paths, those paths are used to instantiate the tunnel LSP.
For TE tunnels with existing instantiated SR-TE LSP, if the head-end detects a discontinuity on any link, the head-end assumes a fault has occurred on that link. In this case, the local repair protection, such as the IP FRR, come in to effect. The IGPs continue to sustain the protected adjacency label and associated forwarding after the adjacency is lost for some time. This allows the head-ends enough time to reroute the tunnels onto different paths that are not affected by the same failure. The head-end starts a tunnel invalidation timer once it detects the link failure to attempt to reroute the tunnel onto other available path-options with valid paths.
If the TE tunnel is configured with other path-options that are not affected by the failure and are validated, the head-end uses one of those path-options to reroute (and re-optimize) the tunnel by instantiating a new primary LSP for the tunnel using the unaffected path.
If no other valid path-options exist under the same tunnel, or if the TE tunnel is configured with only one path-option that is affected by the failure, the head-end starts an invalidation timer after which it brings the tunnel state to ‘down’. This action avoids black-holing the traffic flowing over the affected SR-TE LSP, and allows services riding over the tunnel to reroute over different available paths at the head-end. There is an invalidation drop configuration that keeps the tunnel 'up', but drops the traffic when the invalidation timer expires.
For intra-area SR-TE LSPs, the head-end has full visibility over the LSP path, and validates the path to the ultimate LSP destination. However, for inter-area LSPs, the head-end has partial visibility over the LSP path—only up to the first ABR. In this case, the head-end can only validate the path from the ingress to the first ABR. Any failure along the LSP beyond the first ABR node is invisible to the head-end, and other mechanisms to detect such failures, such as BFD over LSP are assumed.