MPLS Guide Label Distribution Protocol
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5.1.13.1 ECMP vs FRR
If ECMP is enabled, which provides multiple primary next hops for a prefix, LDP FRR
is not used. That is, the LFA next hops are not populated in the RTM and the ECMP
paths are used instead.
5.1.13.2 IGP Shortcuts (RSVP-TE Tunnels)
IGP shortcuts are an MPLS functionality where LSPs are treated like physical links
within IGPs; that is, LSPs can be used for next-hop reachability. If an RSVP-TE LSP
is used as a shortcut by OSPF or IS-IS, it is included in the SPF calculation as a
point-to-point link for both primary and LFA next hops. It can also be advertised to
neighbors so that the neighboring nodes can also use the links to reach a destination
via the advertised next hop.
IGP shortcuts can be used to simplify remote LFA support and simplify the number
of LSPs required in a ring topology.
When both IGP shortcuts and LFA are enabled under OSPF or IS-IS, and LDP FRR
is also enabled, the following applies:
• a FEC that is resolved to a direct primary next hop can be backed up by a
tunneled LFA next hop
• a FEC that is resolved to a tunneled primary next hop will not have an LFA next
hop; it relies on RSVP-TE FRR for protection
5.1.13.3 LDP FRR Configuration
To configure LDP FRR, LFA calculation by the SPF algorithm must first be enabled
under the OSPF or IS-IS protocol level with the command:
config>router>ospf>loopfree-alternates
or
config>router>ospf3>loopfree-alternates
or
config>router>isis>loopfree-alternates
Next, LDP must be enabled to use the LFA next hop with the command
config>router>ldp>fast-reroute.
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