Shared Risk Link Groups; Enabling Disjoint Backup Paths - Alcatel-Lucent 7950 SR User Configuration Manual

Shared Risk Link Groups

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. This requires that the primary LSP already be established
and up since the head-end LER needs the most current ERO computed by CSPF for the primary
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/RSVP task will query again CSPF providing the list of SLRG group numbers to be
avoided. 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/RSVP will keep 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, for
example, the outgoing interface at the PLR the primary path is using. If one or more paths are
found, the MPLS/RSVP task will select one based on best cost and will signal the bypass/detour.
If not and the user included the strict option, the bypass/detour is not setup and the MPLS/RSVP
task will keep 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 path is not guaranteed to be SRLG disjoint from the primary path. This
is because only the SRLG constraint of the outgoing interface at the PLR that the primary path is
using is avoided.

Enabling Disjoint Backup Paths

A typical application of the SRLG feature is to provide for an automatic placement of secondary
backup LSPs or FRR bypass/detour LSPs that minimizes the probability of fate sharing with the
path of the primary LSP
The following details the steps necessary to create shared risk link groups:
•
7950 SR OS MPLS Configuration Guide
(Figure
For primary/standby SRLG disjoint configuration:
 Create an SRLG-group, similar to admin groups.
 Link the SRLG-group to MPLS interfaces.
13).
MPLS and RSVP
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