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MPLS Guide
For a given prefix, the LSP can be used either as a primary next hop or as an LFA next hop,
but not both. If the main SPF calculation selects a tunneled primary next hop for a prefix, the
LFA SPF calculation will not select an LFA next hop for this prefix and the protection of this
prefix will rely on the RSVP-TE LSP FRR protection. If the main SPF calculation selects a
direct primary next hop, the LFA SPF calculation will select an LFA next hop for this prefix but
will prefer a direct LFA next hop over a tunneled LFA next hop.
If the command is used with the lfa-only option, and the rsvp-shortcut command is enabled,
an LSP can be included in the LFA SPF algorithm only. If the advertise-tunnel-link
command is enabled, the tunnel is not advertised as a point-to-point link, is not included in
the main SPF, and is only included in the LFA SPF algorithm.
When the lfa-only option is enabled, the introduction of IGP shortcuts does not affect the
main SPF decision. For a given prefix, the main SPF calculation always selects a direct
primary next hop. The LFA SPF calculation will select an LFA next hop for this prefix but will
prefer a direct LFA next hop over a tunneled LFA next hop.
When the relative-metric option is enabled, IGP will apply the shortest IGP cost between the
endpoints of the LSP plus the value of the offset (instead of the LSP operational metric) when
calculating the cost of a prefix that is resolved to the LSP. The offset value is optional and
defaults to zero. The minimum net cost for a prefix is one (1) after applying the offset. The
Tunnel Table Manager (TTM) continues to show the LSP operational metric as provided by
MPLS; therefore, applications such as BGP and static route shortcuts will continue to use the
LSP operational metric.
The relative-metric option and the lfa-protect or the lfa-only options are mutually exclusive.
An LSP with the relative-metric option enabled cannot be included in the LFA SPF
calculation and the relative-metric option cannot be enabled if the LSP is included in the LFA
SPF calculation when the rsvp-shortcut option is enabled in the IGP.
The relative-metric option is ignored when forwarding adjacency is enabled in OSPF or IS-
IS. In this case, IGP advertises the LSP as a point-to-point unnumbered link along with the
LSP operational metric as returned by MPLS and capped to the maximum link metric allowed
in that IGP.
Both the main SPF and the LFA SPF algorithms use the local IGP database to resolve the
routes.
The no form of this command disables the use of an RSVP-TE LSP by OSPF or IS-IS as a
shortcut or a forwarding adjacency for resolving IGP routes.
For more information on IGP shortcuts and LFA, refer to the 7705 SAR Routing Protocols
Guide, "LDP and IP Fast Reroute (FRR) for OSPF Prefixes" and "LDP and IP Fast Reroute
(FRR) for IS-IS Prefixes".
Default
igp-shortcut — all RSVP-TE LSPs originating on this node are eligible by default as long as
the destination address of the LSP corresponds to a router ID of a remote node
Parameters
lfa-protect — an LSP is included in both the main SPF and the LFA SPF calculation
lfa-only — an LSP is included in the LFA SPF calculation only
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MPLS and RSVP-TE
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