SRLGs for secondary LSP paths apply when LSP redundancy protection is used.
When setting up the secondary path, enable the srlg option on the secondary path to ensure that CSPF includes the SRLG constraint in its route calculation. To make an accurate computation, CSPF requires that the primary LSP be established and in the up state (because the head-end LER needs the most current explicit route object (ERO) for the primary path, and the most current ERO is built during primary path CSPF computation). The ERO includes the list of SRLGs.
At the establishment of a secondary path with the SRLG constraint, the MPLS/RSVP-TE task queries CSPF again, which provides the list of SRLGs to be avoided. CSPF prunes all links having interfaces that belong to the same SRLGs as the interfaces included in the ERO of the primary path. If CSPF finds an eligible path, the secondary path is set up. If CSPF does not find an eligible path, MPLS/RSVP-TE keeps retrying the requests to CSPF.