SRLG Overview
In MPLS traffic engineering, a Shared Risk Link Group (SRLG) is a set of links sharing a common resource, which affects all links in the set if the common resource fails. These links share the same risk of failure and are therefore considered to belong to the same SRLG. For example, links sharing a common fiber are said to be in the same SRLG because a fault with the fiber might cause all links in the group to fail.
An SRLG is represented by a 32-bit number unique within an IGP (OSPFv2 and IS-IS) domain. A link might belong to multiple SRLGs. 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.
When the SRLG is configured, the device uses the Constrained Shortest Path First (CSPF) algorithm and tries to keep the links used for the primary and secondary paths mutually exclusive. If the primary path goes down, the CSPF algorithm computes the secondary path by trying to avoid links that share any SRLG with the primary path. In addition, when computing the path for a bypass LSP, CSPF tries to avoid links that share any SRLG with the protected links.
When the SRLG is not configured, CSPF only takes into account the costs of the links when computing the secondary path.
Any change in link SRLG information triggers the IGP to send LSP updates for the new link SRLG information. CSPF recomputes the paths during the next round of reoptimization.
Junos OS Release 11.4 and later supports SRLG based on the following RFCs:
- RFC 4203, OSPF Extensions in Support of Generalized Multi-Protocol Label Switching (GMPLS).
- RFC 5307, IS-IS Extensions in Support of Generalized Multi-Protocol Label Switching (GMPLS).
 | Note: Currently, the “Fate Sharing” feature continues to be supported with the SRLG feature. |
