Shared Risk Link Groups (SRLGs) is a feature that allows the user to establish a backup secondary LSP path or a FRR LSP path which is disjoint from the path of the primary LSP. Links that are members of the same SRLG represent resources sharing the same risk, for example, fiber links sharing the same conduit or multiple wavelengths sharing the same fiber.
When the SRLG option is enabled on a secondary path, CSPF includes the SRLG constraint in the computation of the secondary LSP path. CSPF would return the list of SRLG groups along with the ERO during primary path CSPF computation. At a subsequent establishment of a secondary path with the SRLG constraint, the MPLS task queries again CSPF providing the list of SRLG group numbers to be avoided. If the primary path was not successfully computed, MPLS assumes an empty SRLG list for the primary. CSPF prunes all links with interfaces which belong to the same SRLGs as the interfaces included in the ERO of the primary path. If CSPF finds a path, the secondary is setup. If not, MPLS keeps retrying the requests to CSPF.
When the SRLG option is enabled on FRR, CSPF includes the SRLG constraint in the computation of a FRR detour or bypass for protecting the primary LSP path. CSPF prunes all links with interfaces which belong to the same SRLG as the interface, which is being protected, that is, the outgoing interface at the PLR the primary path is using. If one or more paths are found, the MPLS task selects one based on best cost and signals the bypass/detour. If not and the user included the strict option, the bypass/detour is not setup and the MPLS task keeps retrying the request to CSPF. Otherwise, if a path exists which meets the other TE constraints, other than the SRLG one, the bypass/detour is setup.
A bypass or a detour LSP is not intended to be SRLG disjoint from the entire primary path. This is because only the SRLGs of the outgoing interface at the PLR the primary path is using are avoided.