The user first enables Loop-Free Alternate (LFA) computation by SPF under the IS-IS routing protocol level or under the OSPF routing protocol instance level.
CLI syntax
config>router>isis>loopfree-alternates
config>router>ospf>loopfree-alternates
config>service>vprn>ospf>loopfree-alternatesThe above commands instruct the IGP SPF to attempt to pre-compute both a primary next hop and an LFA next hop for every learned prefix. When found, the LFA next hop is populated into the RTM along with the primary next hop for the prefix.
Next the user enables IP FRR to cause RTM to download to the IOM or the forwarding engine a LFA next hop, when found by SPF, in addition to the primary next hop for each prefix in the FIB.
CLI syntax
config>router>ip-fast-reroute