Loop-free Alternate (LFA) Protection Support

The following are the details of the Loop-free Alternate (LFA) Protection Support.

1. Prefixes that use one or more SR-TE LSPs as their primary next hops are automatically protected by one of the LFA features, base LFA, remote LFA, or TI-LFA, when enabled on any of the SR-TE LSPs.

2. If the user enables the lfa-only option for a specified SR-TE LSP, then if the application prefix has a single IP primary next hop (no ECMP next hops), it can be protected by an LFA backup that uses an SR-TE LSP.

   Note:

   The LFA SPF calculation cannot check that the outgoing interface of the protecting SR-TE LSP is different from the primary next hop of the prefix. The prefix is still protected by either the ECMP next hops or the LFA backup next hop of the first segment of the protecting SR-TE LSP. This is a difference in behavior with that of an RSVP-TE LSP used with the lfa-only option. In that case, such an LSP is excluded from being used as a LFA backup next hop.

3. Application prefixes that resolve in TTM to an LDP IPv4 prefix FEC, which itself is resolved to one or more SR-TE LSPs, are equally protected either by the SR-TE LSP FRR (1) or the LDP LFA backup using an SR-TE LSP (2).

4. Assume resolution=disabled for one prefix family (for example, IPv6) while it is enabled to sr-te for the other (for example, IPv4). Also, assume a node is resolving an IPv6 prefix and an IPv4 prefix, both of which share the same downstream parent node in the Dijkstra tree. If the IPv4 prefix is protected by the LFA of one or more SR-TE LSP primary next hops (1), the feature supports computing a LFA IP backup next hop for the IPv6 prefix which is resolved to a IP primary next hop. This behavior aligns with the behavior over RSVP-TE LSP used as IGP shortcut for IPv6 and IPv4 prefixes.

5. Assume resolution=disabled for one prefix family (for example, IPv6) while it is enabled to sr-te for the other (for example, IPv4). Also, assume a node is resolving an IPv6 prefix and an IPv4 prefix, both of which share the same downstream parent node in the Dijkstra tree. If the IPv4 prefix resolves to a single primary IP next hop but is protected by the LFA backup next hop that uses an SR-TE LSP (2), the feature does not support computing an LFA IP backup next hop for IPv6 prefix, which then remains unprotected. This is a limitation of the feature that also exists with RSVP-TE LSP used as IGP shortcut for IPv6 and IPv4 prefixes.

   This behavior also applies if the configuration of the resolution command for IPv4 and IPv6 families are reversed.

   If the user enabled the remote LFA or the TI-LFA feature and enabled the use of SR IPv6 or SR IPv6 tunnels as an LFA backup next hop by the LDP IPv6 or IPv4 FEC prefix (LDP fast-reroute backup-sr-tunnel option), the LDP FEC is protected if such a backup SR tunnel is found.