The multi-homed prefix (MHP) LFA feature for IP FRR of IS-IS routes, SR-ISIS tunnel, and SRv6-ISIS tunnel FRR is enabled using the following CLI command.
configure
+--router
+--isis <0..31>
+--loopfree-alternates
+--multi-homed-prefix
+--preference <all, none>
configure
+--router
+--isis <0..31>
+--loopfree-alternate
+--multi-homed-prefix
+--preference <all, none>
When applied to IP prefixes, IP FRR must also be enabled using the following CLI 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 uses 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. Without this feature, the backup path is computed to the ASBR, ABR, or router owner which is the best path for the prefix.
The base LFA algorithm is applied to all intra-area and external prefixes of IP routes (IP FRR), of SR-ISIS node SID tunnels (SR-ISIS FRR), and to SRv6-ISIS remote locator tunnels (SRv6-ISIS FRR), as usual. Then the MHP LFA is applied to improve the protection coverage for external prefixes and anycast prefixes. For external /32 IPv4 prefixes and /128 IPv6 prefixes and for intra-area /32 IPv4 and /128 IPv6 prefixes with multiple owner routers (anycast prefixes), the base LFA backup path, if found, 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 path by setting preference command value to all.
TI-LFA, base LFA, and RLFA (if enabled) are applied to the SR-ISIS node SID tunnels of all intra-area and external /32 IPv4 and /128 IPv6 prefixes as usual, and to all SRv6-ISIS locator tunnels of intra-area and external prefixes of any size.
For node SID SR-ISIS tunnels of external /32 IPv4 and /128 IPv6 prefixes or intra-area /32 IPv4 and /128 IPv6 anycast prefixes, the LFA, TI-LFA, or RLFA 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. Finally, the same preference rule also applies to SRv6-ISIS remote locator tunnels of external prefixes and intra-area prefixes with multiple router owners.
The MHP LFA backup path protects SR-ISIS tunnels and SRv6-ISIS locator tunnels in both algorithm 0 and flexible-algorithm numbers. Therefore, it also extends the protection to any SR-TE LSP, SR-MPLS policy, or SRv6 policy that uses an SR-ISIS SID or an SRv6-ISIS SID of those same prefixes in its configured or computed SID list.