Example
In Fig.1 (right), there is a primary path (Label Switched Path, or LSP) from A to E via B and D. The traffic of customers connected to A and E will take this path in the normal operation. There is also a secondary path (LSP) from A to E via C. This path can be either pre-signaled or not. For the primary LSP, FRR (Fast ReRoute) is enabled. Once enabled, the other network elements on the LSP will know that FRR is enabled. Let's assume there is a break between D and E. D will immediately know this and it will inform B and A. For A to know that there is a failure between D and E, it takes a while. Since D gets to know immediately about the failure and FRR is enabled on the LSP, it uses the detour path D-C-E to get rid of the failure immediately and traffic will continue to flow along that path. This takes less than 50ms. Once the secondary LSP is up, traffic is switched to the secondary LSP and detour path is turned down.
Read more about this topic: MPLS Local Protection
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