MPLS and RSVP-TE
210
MPLS Guide
© 2022 Nokia.
Use subject to Terms available at: www.nokia.com
3HE 18686 AAAB TQZZA
srlg
Syntax [no] srlg
Context config>router>mpls>lsp>secondary
Description This command enables the use of the SRLG constraint in the CSPF computation of a
secondary path for an RSVP-TE LSP at the head-end LER. When this feature is enabled,
CSPF includes the SRLG constraint in the computation of the secondary RSVP-TE LSP path.
CSPF and SRLGs for Secondary Paths
CSPF requires that the primary LSP be established already and in the up state, since the
head-end LER needs the most current ERO computed by CSPF for the primary path and
CSPF includes the list of SRLGs in the ERO during the CSPF computation of the primary
path. At a subsequent establishment of a secondary path with the SRLG constraint, the
MPLS/RSVP-TE task queries CSPF again, which provides the list of SRLG numbers to be
avoided. CSPF prunes all links with interfaces that belong to the same SRLGs as the
interfaces included in the ERO of the primary path. If CSPF finds a path, the secondary path
is set up. If CSPF does not find a path, MPLS/RSVP-TE keeps retrying the requests to CSPF.
If CSPF is not enabled on the LSP (using the lsp lsp-name>cspf command), a secondary
path of that LSP that includes the SRLG constraint is shut down and a specific failure code
indicates the exact reason for the failure in the show>router>mpls>lsp>path>
detail output.
Primary Path and Secondary Path Behavior
At initial primary LSP path establishment, if the primary path does not come up or is not
configured, the SRLG secondary path is not signaled and is put in the down state. A specific
failure code indicates the exact reason for the failure in the show>router>mpls>lsp>path>
detail output. However, if a non-SRLG secondary path was configured, such as a secondary
path with the SRLG option disabled, MPLS/RSVP-TE task signals it and the LSP uses it.
As soon as the primary path is configured and successfully established, MPLS/RSVP-TE
moves the LSP to the primary path and signals all SRLG secondary paths.
Any time the primary path is reoptimized, has undergone a make-before-break (MBB)
operation, or has come back up after being down, the MPLS/RSVP-TE task checks with
CSPF to determine if the SRLG secondary path should be resignaled. If the MPLS/RSVP-TE
task finds that the current secondary path is no longer SRLG disjoint — for example, the path
became ineligible — it puts the path on a delayed make-before-break immediately after the
expiry of the retry timer. If MBB fails on the first try, the secondary path is torn down and the
path is put on retry.
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