Shared Risk Link Groups (SRLG) is a feature which allows the user to establish a backup secondary LSP (label switched path) path or a FRR (fast-reroute) LSP path which is disjoint from the path of the primary LSP. Links which are members of the same SRLG represent resources which share the same risk. For example, fiber links sharing the same conduit or multiple wavelengths sharing the same fiber.

SRLG groups are used to determine which links belong to the same SRLG. To advertise SRLG, the information is part of the IGP TE parameters in an opaque LSA (link state advertisement). It is advertised in a SRLG sub-TLV (type 16) of the existing Link TLV in OSPF.

The SRLG of a path in a label-switched path (LSP) is the set of SRLGs for all the links in the path. When computing the secondary path for an LSP, it is preferable to find a path such that the secondary and primary paths do not have any links in common in case the SRLGs for the primary and secondary paths are disjoint. This ensures that a single point of failure on a particular link does not bring down both the primary and secondary paths in the LSP.

Backup tunnel path selection with SRLG is done so that a backup tunnel can avoid using links that are in the same SRLG as the interfaces it is protecting. Otherwise, when the protected link fails the backup tunnel fails too.