This feature is enabled by configuring both the segment routing and forwarding adjacency features within an IS-IS instance in a multi-topology MT=0.
Both IPv4 and IPv6 SR-ISIS tunnels can be resolved and further tunneled over one or more RSVP-TE LSPs used as forwarding adjacencies.
This feature uses the following procedures.
The forwarding adjacency feature only advertises into IS-IS RSVP-TE LSPs. SR-TE LSPs are not supported.
An SR-ISIS tunnel (node SID) can have up to 32 next hops, some of which can resolve to a forwarding adjacency and some to a direct IP link. When the router ecmp value is configured lower than the number of next hops for the SR-ISIS tunnel, the subset of next hops selected prefers a forwarding adjacency over an IP link.
In SR OS, ECMP and LFA are mutually exclusive on per-prefix basis. This is not specific to SR-ISIS but also applies to IP FRR, LDP FRR, and SR-ISIS FRR. If an SR-ISIS tunnel has one or more next hops that resolve to forwarding adjacencies, each next hop is protected by the FRR mechanism of the RSVP-TE LSP through which it is tunneled. In this case, LFA backup is not programmed by IS-IS.
If an SR-ISIS tunnel has a single primary next hop that resolves to a direct link (not to a forwarding adjacency), base LFA may protect it if a loop-free alternate path exists. The LFA path may or may not use a forwarding adjacency.
IS-IS does not compute a remote LFA or a TI-LFA backup for an SR-ISIS tunnel when forwarding adjacency is enabled in the IS-IS instance, even if these two types of LFAs are enabled in the configuration of that same IS-IS instance.