The multi-homed prefix (MHP) LFA feature for IP FRR of OSPF routes and for SR-OSPF FRR is enabled using the following CLI command.
configure
+--router
+--ospf <0..31>
+--loopfree-alternates
+--multi-homed-prefix
+--preference <all, none>
configure
+--router
+--ospf <0..31>
+--loopfree-alternate
+--multi-homed-prefix
+--preference <all, none>
When applied to IP prefixes, IP FRR must also be enabled using the following command, which allows the programming of the backup paths in the FIB.
configure
+--router
+--ip-fast-reroute
configure
+--routing-options
+--ip-fast-reroute
This feature makes use of the multi-homed prefix model described in RFC 8518 to compute a backup IP next hop via an alternate ABR or ASBR for external prefixes and to an alternate router owner for local anycast prefixes.
The base LFA algorithm is applied to all intra-area and external IP prefixes (IP FRR) and SR-OSPF node SID tunnels (SR-OSPF FRR), as usual. Then the MHP LFA is applied to improve the protection coverage for external prefixes and anycast prefixes. For external /32 prefixes and intra-area local /32 prefixes with multiple owner routers (anycast prefixes), the base LFA backup path, if found, is preferred over the MHP LFA backup in the default behavior with the preference command set to a value of none. The user can force the programming of the MHP LFA backup by setting the preference command value to all. The algorithm details are described in RFC 8518 MHP LFA for OSPF.
After the IP next-hop based MHP LFA is enabled, the extensions to MHP LFA to compute an SR-TE repair tunnel for an SR-OSPF tunnel are automatically enabled when the following CLI command is configured to enable Topology-Independent Loop-Free Alternate (TI-LFA) or Remote Loop-Free Alternate (RLFA). The algorithm details are described in 7750 SR and 7950 XRS Segment Routing and PCE User Guide, section LFA solution across IGP area or instance boundary using SR repair tunnel for SR OSPF. The computation reuses the SID list of the primary path or the TI-LFA or RLFA backup path of the alternate ABR, ASBR, or alternate owner router.
configure
+--router
+--ospf <0..31>
+--loopfree-alternates
+--remote-lfa
+--ti-lfa
configure
+--router
+--ospf <0..31>
+--loopfree-alternate
+--remote-lfa
+--ti-lfa
TI-LFA, base LFA and RLFA (if enabled) are applied to the SR-OSPF tunnels of all intra-area and external /32 prefixes as usual. For node SID SR-OSPF tunnels of external /32 prefixes or intra-area /32 anycast prefixes, the LFA, TI LFA, or RFLA backup path is preferred over the MHP LFA backup path in the default behavior with the preference command set to a value of none. The user can force the programming of the MHP LFA backup by setting the preference command value to all.
The MHP backup path protects SR-OSPF tunnels in both algorithm 0 and flexible-algorithm numbers. It also extends the protection to any SR-TE LSP or SR policy that uses an SR-OSPF SID of those same prefixes in its configured or computed SID list.