Once FRR is triggered, a make-before-break operation is performed to re-establish the primary path. When an established path attempts to reroute onto a new path, the ingress device continues to maintain existing paths and allocated bandwidths, ensuring that the existing path is not prematurely torn down and allowing the current traffic to continue flowing while the new path is set up.
The following diagram shows an example of a primary LSP between A-B-C and backup over bypass tunnel on the path A-D-C. The primary LSP is configured without a strict path. When the interface between A-B goes down, the global revertiveness feature triggers a new LSP on the path A-E-C. The traffic is shifted to the new instance and old instance is torn down.
When the primary LSP is triggered with strict a path (A-B-C), after global revertiveness is triggered, a new instance tries the same path given in the strict path. In the diagram below, the new instance also tries to come up in the path A-B-C.
