Ecmp Vs Frr; Igp Shortcuts (Rsvp-Te Tunnels); Ldp Frr Configuration - Nokia 7705 SAR-W Series Manual

MPLS Guide
5.1.13.1
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 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:
5.1.13.3
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:
Next, LDP must be enabled to use the LFA next hop with the command
config>router>ldp>fast-reroute.
3HE 18686 AAAB TQZZA

ECMP vs FRR

IGP Shortcuts (RSVP-TE Tunnels)

• 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

LDP FRR Configuration

config>router>ospf>loopfree-alternates
or
config>router>ospf3>loopfree-alternates
or
config>router>isis>loopfree-alternates
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