Shared Risk Link Groups; Srlgs For Secondary Lsp Paths - Nokia 7705 SAR-W Series Manual

MPLS and RSVP-TE

3.8 Shared Risk Link Groups

A shared risk link group (SRLG) represents a set of interfaces (or links) that share
the same risk of failing because they may be subjected to the same resource failures
or defects. Two examples where the same risk of failure exists are fiber links that
share the same conduit, and multiple wavelengths that share the same fiber.
SRLGs are supported by both LSP redundancy protection and FRR protection.
SRLGs allow the user to prepare a detour route that is disjoint from the primary LSP
path. See
The SRLG feature ensures that a primary and secondary LSP path, or a bypass
tunnel or detour LSP path, do not share SRLGs. That is, they do not share the same
sets of links that are considered to have a similar (or identical) chance of failure.
To use SRLGs, the user first creates an SRLG by assigning one or more routers to
the SRLG. Then, the user links the SRLG to an MPLS interface and enables the
SRLG feature on the LSP path. SRLGs cannot be assigned to the system interface.

3.8.1 SRLGs for Secondary LSP Paths

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.
80
Disjoint and Non-disjoint
© 2022 Nokia.
Use subject to Terms available at: www.nokia.com
Paths.
MPLS Guide
3HE 18686 AAAB TQZZA