MPLS and RSVP-TE
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MPLS Guide
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3.8.4 Enabling Disjoint Backup Paths
A typical application of the SRLG feature is to provide automatic setup of secondary
LSPs or FRR bypass or detour LSPs, in order to minimize the probability that they
share the same failure risks with the primary LSP path (see Figure 10 and Figure 11).
Figure 10 illustrates SRLG when LSP redundancy is used, where SRLG_1 contains
the interfaces that define links A-B, B-C, and C-D. The primary path uses these links
to connect node A to node D. In the event of a failure along the primary path, the
secondary path cannot use any of the links in SRLG_1 and takes the path from node
A to nodes E, F, G, H, J, and D.
Figure 11 illustrates SRLG when FRR bypass is used, where SRLG_1 is the same
as in Figure 10. Since FRR bypass is used, the following possible reroutes may
occur, depending on where the failure occurs:
• if node B fails, the bypass is from node A to nodes E, F, G, H, and C
• if node C fails, the bypass is from node B to nodes F, G, H, J, and D
• if link C-D fails, the bypass is from node C to nodes H, J, and D
The SRLG feature is supported on OSPF and IS-IS interfaces for which RSVP-TE is
enabled.
The following steps describe how to enable SRLG disjoint backup paths for LSP
redundancy and FRR.
LSP Redundancy for Primary/Secondary (standby) SRLG Disjoint Configuration
• Create an SRLG-group (similar to creating an admin group).
• Link the SRLG-group to MPLS interfaces.
• Configure primary and secondary LSP paths, and enable SRLG on the
secondary LSP path. The SRLG secondary LSP paths will always perform a
strict CSPF query.
The setting of the srlg-frr command is irrelevant in this case (see the srlg-frr
command).
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