LFA Protection Option Applicability

Depending on the configured options of the loopfree-alternates command, the LFA SPF in an IGP instance runs algorithms in the following order:

  1. Computes a regular LFA for each node and prefix. In this step, a computed backup next-hop satisfies any applied LFA policy.

    This backup next-hop protects that specific prefix or node in the context of IP FRR, LDP FRR, SR FRR, and SR-TE FRR.

  2. Follows with the TI-LFA, if the ti-lfa option is enabled for all prefixes and nodes, regardless of the outcome of the first step.

    A prefix or node for which a TI-LFA backup next-hop is found overrides the result from the first step in the context of LDP FRR, if the LDP fast-reroute backup-sr-tunnel option is enabled, in SR FRR and in SR-TE FRR.

    With SR FRR and SR-TE FRR, the TI-LFA next-hop protects the node-SID of that prefix and any adjacency-SID terminating on the node-SID of that prefix.

    The prefix or node continues to use the backup next hop found in Step1 in the context of LDP FRR (if the LDP fast-reroute backup-sr-tunnel option is disabled), or in IP FRR.

  3. Runs remote LFA only for the next-hop of prefixes and nodes which remain unprotected after Step1 and Step 2 if the remote-lfa option is enabled. A prefix or node for which a remote LFA backup next-hop is found uses it in the context of LDP FRR, when the LDP fast-reroute backup-sr-tunnel option is enabled, in SR FRR and in SR-TE FRR.

To protect an adjacency SID, the LFA selection algorithm uses the following preference order:

  1. Adjacency of an alternate parallel link to the same neighbor. If more than one adjacency exists, select one as follows:

    1. adjacency with the lowest metric

    2. adjacency to the neighbor with the lowest router ID (OSPF) or system ID (IS-IS), and the lowest metric

    3. with the lowest interface index and the lowest router ID (OSPF) or system ID (IS-IS)

  2. An ECMP next hop to a node-SID of the same neighbor that is different from the next hop of the protected adjacency. If more than one next hop exists, select one as follows:

    1. next hop with the lowest metric

    2. next hop to the neighbor with the lowest router ID (OSPF) or system ID (IS-IS) if same lowest metric

    3. next hop to the lowest interface index if same neighbor router ID (OSPF) or system ID (IS-IS

  3. LFA backup outcome of a node SID of the same neighbor, in the following preference order:

    1. TI-LFA backup

    2. LFA backup

    3. RLFA backup

Parent topic: TI-LFA Link-Protect Operation